What do you think of these spells? It takes 2 spells cast to become a creature, several rounds to attack enemies while polymorphed and then a third spell to go back to natural form again. Is it worth it?

Polymorph Bears: The Brown Bear seems to be better than the Black Bear. The only difference is AC; the black bear has 7 AC and the brown bear has 6 AC. They both have 3 attacks per round with +1 claws (1d6), and are always hasted, so they get 4 attacks/round. Since they have 18/00 STR, they still do a lot of damage on average, like the Ogre. Always choose a bear over the Ogre if improved hasted though.

Ogre: He is not much different from a bear. He has 1 attack/round but a very powerful weapon (4d6) and AC 5 and 18/00 STR. Since he is always hasted, he has 2 attacks/round. On average, the Ogre should do as much damage as a bear. The damage dealt is less reliable though, and his weapon is not enchanted.

Flind: Has a +1 weapon that deals 1D10 piercing damage with +1 fire damage. He is an expert with this weapon too, giving him an additional attack/round and a small to-hit bonus (+2 in total). With polymorph auto-haste, the attack per rounds is 3. He has a lower CON, which could mean lower HP, but makes up for it with a Base AC of 1 and high dexterity so he ends up with -2 AC.

Spider: Deals 1D4 damage + poison (not enchanted) and has 5 attacks/round (would be 4 without haste). The AC is very good at -3. There is no bonus in to-hit though, and the CON is only 9, but like the Flind, the AC bonus can make up for it.

Mustard Jelly: The 100% magic resistance, and 30% immune to slashing, 60% immune to crushing, 85% immune to missiles, 100% immune to piercing, and 0 AC makes, this creature quite useful at times. The attack is poor, at 1d6 (+1 weapon), but it slows opponent too (good for defense). The CON is poor as well, but the absorption and low Base AC can make up for that. This polymorphed mustard jelly is very different from the one you can turn into with the cloak of the sewers.

Winter Wolf: Pretty average. The AC is 2, number of (piercing) attacks with haste is 2 (1d10 +1 weapon), no to-hit bonus, and receives a penalty to HP. Cold immunity and 50% electricity resistance helps though.

In summary, use the bears or ogres if you're not attacked and want to deal as much damage as you can. Use the Flind or Spider if you want AC bonus, use the Jelly if you want magic resistance and absorb damage. The wolf does not seem very useful to me. I'd only use it over the mustard jelly for the speed in situations where you need extra cold or electricity protection.

The alternative to not using polymorph spell can of course work much better for the party, but I still think it's not a bad spell, at least in the beginning of the game when the characters are more vulnerable. The best thing is that it's convenient, just like the fighter class is convenient. You don't need to spend too much time thinking about what to do, just shift into a form that is suitable for the situation and attack. I don't think that magical weapons are that different from polymorph either. They'll give you some bonus to-hit and when you still can cast spells, that should make you equally effective as a fighter.

Shapechange spell on the other hand seems much worse.

Iron Golem: A more powerful version of the Mustard Jelly. Great resistances, immunity to +2 weapons and backstab. Has a +4 weapon and 24 STR, but only 1 attack/round and a penalty to HP.

Fire/Earth Elemental: Similar to the Iron Golem, but worse. Not sure why you would want to turn into one of these instead of the Iron Golem.

Mindflayer: This one is more about killing enemies quickly with intelligence drain of 5/hit, 4 attacks per round, but with a -1 to to-hit you need to choose target carefully and probably make some preparations before turning into a mindflayer, like reducing the enemy AC and so on. It's vulnerable to physical attacks with 5 AC but resist magic well with 90% magic resistance. It also has 1 psionic attack available.

Greater Wolfwere: Well rounded with decent 1D12 slashing weapon (+1), immunity to +1 weapons, +3 to-hit, +7 damage bonus, 3 attacks/round, regeneration 8 hp/second, -2 AC and resist 50% elemental magic. It is like a black bear but with great defense.

Giant Troll: Much worse than the Greater Wolfwere, but is immune to paralyze, sleep and stun, and it is fast. The enemy unfortunately don't require fire or acid to kill it, unlike the enemy trolls and the regeneration rate is only 1 hp/sec.

For a level 9 spell, you'd expect more, because once a mage can cast level 9 spells, it will have plenty of powerful options already. Still, it is nice to have a permanent immunity to most weapons and the mind flayer opens up some interesting options. A popular tactic for example is to cast a time stop and then to turn into a mindflayer and kill everything in 1 round or less (everything hits in time stop). For convenience, the greater wolfwere is pretty decent too. Then you just attack one enemy at a time with everyone until they are all dead and don't worry too much about defense, and against Beholders and other magical creatures, the Iron Golem should be pretty good.

Polymorph has one (good) use in my opinion, and that's the above. It's should be renamed Lich-bane (of course, I'm joking). Watch as the Lich wastes all of it's spells on the jelly, then mop up. 'nough said.

Yes, that's the most popular option. Still, I think it's not a bad option to turn into a spider for example when you need extra AC, because the spider starts with -3 AC (Base AC 3, then -4 from item and -2 from 16 DEX), at least for a mage with poor dexterity or no good shield. You don't actually need that extra AC, but it's there if you are out of stoneskins or something. Try it in the beginning of the game for example where enemies have 17 THAC0 and 5% chance to hit a spider with -3 AC. If the mage had 18 DEX and shield activated instead, the AC would be 0 and they would hit 20% of the times, meaning it would take 4 times longer to kill a spider than an 18 DEX shielded mage on average. An extra blur spell would give the same result too for a mage with 18 DEX, so maybe you just want to go with the 18 DEX mage instead. Usually, I only care about intelligence and wisdom when I create a mage though. Later in the game, once you have spirit shield and tenser's transformation, turning into a spider becomes less of an attractive option, but it all depends on which spells you prefer to memorize. One polymorph spell is less expensive than two other spells, and may last longer as well.

Honestly I've never been much enamored with any of the shapechange spells, except for the Iron Golem which is amazing. The problem is that it's a 9th level spell, so it's competing against a lot of very useful spells for a very limited number of slots. If you're a fighter-mage with good thaco, you're much better off with Energy Blades or BBoD than being a golem. If you're a straight mage or a thief/mage, you're better off using direct damage or a Planetar.

In PnP, Polymorph spells are extremely useful for various tasks that don't necessarily include combat, but for Baldur's Gate they're pretty lackluster.

I agree. The golem has great resistances and deals the most damage despite only doing 1 attack/round, but it too receives a hit point penalty too. A mage with already low hit points can't really afford losing 10 or 20 hit points. The developers shouldn't have messed with the hit points in any of these creatures. A normal iron golem has 80 hit points. That seems reasonable and not at all unbalanced for a level 9 spell.

I've been analyzing this some more and can see that the jelly absorption is quite good most of the times. You can view the absorption as as a HP extender. For example, if a jelly is hit by a slashing weapon, they have about 43% more hit points. If the jelly is hit by a crushing weapon, the hit points has increased 2,5 times and if hit by missiles, the hit points have increased 6,67 times. Let's say that the polymorphed jelly has 25 HP, then the missile absorption will be like increasing the mage's HP by about 120-140 HP, the crushing absorption by about 20-40 HP, and the slashing absorption will increase/decrease the mage's HP by about -10 to 10 depending on the constitution of the mage. Basically, the only time where the jelly is not a good option is when enemies are using swords or axes. Possibly, 20-40 HP is not worth it either since you lose the ability to cast spells while you're a jelly.

An AC bonus by 1 could be viewed as extending the the HP similar to absorption (expected average), and this depends on THAC0-AC:
2-10% if THAC0-AC is in the range [2,10]
10-25% if THAC0-AC is in the range [11,17] and
33-100% if THAC0-AC is in the range [18,20].

Then you can multiply these percentages by the AC difference between the creature and the mage to see how good the AC bonus actually is. For example, one could see that if THAC0-AC=20, the spider extends HP by 4 times compared to the jelly (300%). This is better than the 2,5 times extension received from crushing absorption or 1,43 times received from slashing absorption. Still, it's highly unlikely that this will happen I think, so in general, the jelly is almost always better against piercing, missile and crushing than the mage or spider. Against slashing weapons, the spider is better if THAC0-AC (jelly AC) is larger than 11 but the AC improvement from mage to spider needs to be at least 3 or more if constitution is >14.

As you can imagine, Tenser's Transformation is often a better option for improving your odds to survive since you get these AC bonuses (-4 AC) on top of the double HP. All you need to do is get a decent AC difference between the jelly, and AC low enough to be good against enemy THAC0. My experience with Tenser's Transformation is that the mage is on par with a Two-handed wielding Paladin or something like that (without the extra attacks or saving throws). That is pretty good, at least in combination with other spells.