I feel like going through and commenting on some of these points:
- Aerie: characterwise she is a bit whiny. Party-wise her progression is sluggish.
She is able to cast both cleric and mage spells, which allows you to do interesting things with sequencers. For example, have her polymorph into a form that gets a lot of attacks (like a spider), then launch a minor sequencer with Draw Upon Holy Might. This can get her lots of attacks and 19+ strength, a pwerful combination. (Use Mirror Image/Stoneskin to keep her alive.) In any case, if you get a Minor Sequence scroll, she is the best character (not counting custom characters) to be given that spell.
Also, she can use Vhailor's Helm. Give her that helm and a Scroll of Limited Wish, and you can get wishes early without using up the scroll; this is particularly useful for dealing with level-draining vampires. Also, she has enough wisdom to get all the good effects from that spell.
- Imoen: hard to get. Mediocre stats.
Assuming you are referring to the Spellhold version, she is actually easy to get, in the sense that you are guaranteed to get her unless you use a glitch to skip that part of the game.
Now, if you (for some reason) mean the Irenicus's Dungeon version, then you have a character who is hard to get (or, rather, is hard to keep past the beginning). This version has a lot of immunities thanks to Imoen's Belt, but will leave your party when her HP gets down to 1, and can't be re-recruited afterwords.
There's also the tutorial version, but I don't know of a way to get her into Shadows of Amn proper; this version is actually very good at setting traps. (You can get her in ToB by importing a tutorial save, but that may not be worth it.)
- HaerDhalis: hard to get, and his class is weak. His personalequipment is also overspecialized. Not have any extra sidequest.
He gets a higher caster level than a mage with equivalent XP gets; this makes him a good candidate for spells that scale with level (if you find a Remove Magic scroll, give it to him).
There's also the trick of using Mislead with Improved Bard Song; even without stacking it, it enables you to have the song active while still having him available to fight.
Also, if you're not playing the EE, the ability damage from his swords can be used to kill enemies with minimum HP items; in other words, he can kill the game's equivalent of essential NPCs. (Note that this can break the main quest if you're not careful.)
- Jan: strong caster, acceptable thief.
As a caster, Aerie is better because she has more types of spells; both characters progress in arcane casting at the same rate.
(Of course, Jan does have his advantages, like access to thief skills and HLAs.)