I'm playing SoU as a sorcerer, getting close to level 6 and thus my first 3rd level spell. I'm guessing fireball is a no-brainer, presumably followed by haste?

Anyway, I am a bit confused as to whether there are actually any useful 2nd level spells besides Melf's Acid Arrow. At first I took combust, because I wasn't paying attention and didn't notice the "range: touch" bit. So I swapped it out for Melf's and Gedlee's Electric Loop, because I figured the latter would be good for damaging more than one enemy... but so far I haven't managed to do anything other than damage one enemy and myself. Am I doing it wrong or is it just bugged? Long-term I guess I'll be devoting 2nd level spells to stuff like Cat's Grace and Eagle's Splendor, but is there anything there which is more useful than magic missile for a level 5 caster?

Buff spells are great for your pet or henchman.

I tend to play more defensive with 2nd level spells b/c a good crossbow can do a ton more damage with just a little time compared to the damage that the spells do (besides the very useful melf's).

I recommend looking at:
Ghostly Visage -- great for early game. Creatures have a hard time getting through it.
Resist elements - Great for wizard-battles
Summon Creature -- keep enemies at a distance with all the summons and henchies you can get. Summon creature provides the most damage per spell than any other spell (this was fixed in 3.5 when they limit your spawns from 24 hours to just a few short rounds)

I've also had limited success with web and balagorn's horn (I used horn when I played a bard -- they get it much earlier). They help a bit, but not nearly as much as the others.

I wanted to emphasize on this part. Summon creature spells are very-very useful in NWN1 (and in D&D games in general). You don't want to miss on those. They are very useful especiaily in hard encounters, where you can send them to opponents to wear them down.

I would suggest prioritizing Summon of current spell level -> 1 most potent Breach-like effect (the one which strips opponent from magic and other defenses or puts some nasty debuff on them) of current spell level -> 1 Most potent Buff of current spell level -> Direct Damage spell. As sorcerer, you don't have to manage spells same way as mage, thus you can choose a lot of fluffy spells for low levels. Higher level spells will be no-brainer.

I didn't play NWN1 for quite a time, but IIRC you should have access to Blur and Mirror Image as 2nd level spells. Those are 2 great buffs which complement each other very well.

You might also want to have Protection from Alignment (Evil) 1st level spell if you don't have it yet, otherwise you will have problems with high-end summons.

Not to disrespect my fellow posters here, but I personally think you should stick with the Evocation and Conjuration Schools. Later in the game the Abjuration, or Buff spells, become nigh useless in my honest opinion.(I played as both a Wizard and Sorcerer in D&D for about eleven years now).

However, it is good to balance out, a few offensive spells, a few defensive, and several Conjure Monster spells.

Of course, in NWN1, the Sorcerer can switch spells out at level-up, so you can choose buff spells early on and then get rid of them later when you're levelling up.

Protection from Alignment is very good to have; one of the easiest ways to get immunity to mind-affecting spells. Color Spray or Sleep are fantastic at low-levels, but they quickly become obsolete against more advanced enemies so you'll want to trade them out eventually. Grease isn't a bad spell for preventing enemies from closing the distance with you. At 2nd level, ensure you have the ability to "see invisibility". Cloud of Bewilderment and Web are great debuffs. Death Armor can be a useful way to deal with low-damage enemies or those with SR. It stacks with the more powerful acid sheath and elemental shield, too, so you can get some truly outrageous damage feedback.

The summon spells are an interesting bunch. The biggest caveat is that you will actually receive less experience when you have a summoned creature out. However, with an unlimited duration the spell is an incredibly reliable way to get blockers.

Right, I forgot to mention that I am intentionally avoiding the summon monster spells due to the (IMHO unreasonable) XP effects.

NWN1 has scaling in original campaign (people say there is no scaling in other 2 main campaigns, but with char import, there are no EXP problems at all). This was a bad news to me when I was finishing 1st chapter and game started throwing high level mobs at me as punishment for completionism. You might rethink usefulness of XP reduction or might find yourself in situation when story boss is surrounded by mobs far more powerful than actual boss.

Edit: Seeing SotU being mentioned by OP, there are so many creeps in SotU, you will definitely have no EXP problems with or without summons. Don't forget that the higher level you are, the less EXP (down to 0) you get from mobs according to game mechanics. No-summons make you reach that point much faster. Mobs higher levels than you give bonus exp, so in the end you will be at around same level with or without summons.

Never really played a Sorc in NWN xD I dual classed a fighter/thief

My favorite was always Bard with 4 levels of FTR by level 20. Probably at least half of the hundreds of characters I made were FTR/Bard of some sort. I really enjoyed the FTR/Rogue combo as well (FTR/Rogue/CoT is especially awesome). But I played everything except Monks (and Druid only very rarely). The ability of the SOR to switch out spells was a huge plus for the class.

I've played all the classes. My favorite was my Monk/Wizard. A Spell casting, fist throwing doom fighter.