I'm on topamax (For PKD, not epilepsy), and I the only side effect I'm having is a slight, constant weight gain. It's really annoying, as it won't stop, no matter what I do, and it's the only medication I have left to try, and the only one with success and no major side effect.
I guess some people just react differently. I've tried loads of different antiepileptics and it's either been no effect, side effect or side effect and good effect. I'm not really that lucky when it comes to medication.