I never gained much weight on Seroquel. In fact, there have been periods where I actually lost weight. On 600mg and up, I did get a little more hungry, but it was never anything extreme, and I did not have to work out a lot to maintain my weight.
Overall, Seroquel has definitely helped me. My dose is always being tweaked according to my episodes. Sometimes higher doses are hard for me to tolerate, but lowering it a little bit helped avoid some of the serious side effects.