Yes, I have been on several AP's.
The worst weight-gainer for me was Seroquel. I started out weighing 110lbs and, seven years later, weighed 250lbs. I never, never should have agreed to stay on that awful medication - which was never helpful, anyway.
As soon as I stopped taking Seroquel I lost weight. 16lbs in 16 days. And I wasn't trying to lose weight; I just stopped feeling insanely hungry, so wasn't eating large portions, etc.
Then my pdoc prescribed Trilafon, which is amazingly helpful for me. Unfortunately, I'm sure I've gained the 16lbs back - and maybe more. At this point, however, I'm staying on the Trilafon because it truly is helpful for me.