I was already overweight when I started taking it, and didn't notice a huge difference. Maybe 5 lbs? However, I'm trying to lose weight now and it's JUST not coming off. I think the Risperdal may be the culprit - I have to fight for every pound I lose, which hasn't been the case before.
If you're concerned about it, talk to your doctor, but if you need the medication, it really works. For me, it took awhile before settling on this one, but in the end I've been really happy with it. The weight issue is something I willing to deal with because it makes a difference, KWIM?
Editing because I just read through the answers - I didn't address other side effects because you didn't ask about them, but will address the tremors. You'll likely have them, but they get a LOT better after a few weeks. I've been on it several years now and I still shake occasionally, but I honestly don't notice it much anymore. There are other atypical anti-psychotics out there, but this one is relatively mild and most of the side effects fade away pretty quickly (except the darn weight issue). Make sure you drink A LOT of water, it will help.
I want to stress again that if you're uncomfortable with it, talk to your doctor, and ask them about your concerns. It's important to be on the same page as to your treatment options.