I restarted the game to get Imoen at a lower level so that she will have nearly the same experience as the rest of the party. I am realizing it is impossible unless you do the default party (Minsc, Aerie, Jaheria). The reason is I want to do the bard quest because it takes a long time for the stronghold story to pay off. But that alone, along with the chapter, will outpace the experience of a rescued Imoen. Basically, getting Valygar, or some other character means you cannot have Imoen with a close number of exp.
It's very messed up.
Can someone spoil me whether Imoen is worth having, in terms of dialogue and special abilities gained?
Are there any NPCs that don't actually require you do their quests? Maybe Mazzy? Because you have to do them before you go to the asylum or they leave (I assume).

after you save her she gets 6 level 6 spells! SHE is the best mage in the game after a while!

Jon, what are you talking about ? I had Imoen
to the very end she never gets anything close
to 6 level 6 spells.

I can't be sure about this one, but I think that having Imoen in my party may have ruined the romance that I had going. I don't know if it's possible or not, but I can't think of anything else that may have caused the romance to stop. Everything was going fine, then Imoen shows up and shortly after the romance was completely dead.

Also, she sucks as a thief but as a Mage she is pretty good (as far as NPC's go)

-"Easy"

Personally I hate that they locked her thieving skills in BG 2. I mean in the first one I kept her as a thief, and she totally rocked. Now her thieving skills are pathetic. If you have her for a thief in the party, I'd get rid of her and use Jan instead. As a mage well she suffices I guess, though I prefer Aerie as a mage over Imoen, plus Aerie has the advantage of cleric spells as well.

Please post your spoilers. Does Imoen get any special powers from the experiments?
Does Imoen say interesting things that should not be missed? If not, there's no reason not to keep Nalia who is Imoen's identical twin in stats (except even better), who will have more exp, and has a pretty powerful ring on her finger.