I've been on wellbutrin for 6 months or more now. Even at 300 mg, I noticed no effects at all, of any sort - even improving my mood.
My doc and I added lexapro to the mix, and over the next several weeks that had a noticeable effect. A month or two later I tried stopping the wellbutrin, but that resulted in a really bad couple of weeks. Weird.
I don't think the mix is working as well now, though, so it's probably time to try something else. :|