Unlike BG/BG2, druids in this game, if you have HOW, are excellent, and in my view their spells rival those of mages. Indeed, I think they may even be a bit better, though that is, of course, wholly subjective. I really don't think I could live without Entangle combined with Spike Growth, and Spike Stones.
So, the question is, which character class do you view as indispensable? Clearly, everyone's play style is different, so no doubt we'll all have differing views on the subject