[Nerull]

To be honest, Imoen has been in just about any party I have made (except the all evil that I am playing off and on). I don't use backstabbing and traps too much, so she handles the two areas that require a thief (locks and traps). If you Baldurdash your game, it gives her extra thief points, making her much more useful (100% in open locks, 95% in find traps). Combine that with the fact that she advances at the same rate as a single class in mage, and she is very useful. I usually go very early to get her back (first opportunity), so she does not lag too far behind.

My first game, I put Imoen, Nalia, and Aerie in the same party. All were good in their specific areas. Imoen was the party thief, who also was #2 in the mage department. Aerie was backup cleric, and also backed up mage quite nicely. Nalia was a straight mage in my party; I did not once use her thieving skills (except for weapons, of course). Heavy on magic, to be sure, but still very good.

I guess it all comes down to your preference. I have always seen her as a strong member of the party, but others may disagree.