I'm on Celexa now, but I've got that for depression as well as IBS. Before I started on that, I was on a low dose of another one, but I can't remember which.
I found that this type of med helped with the cramps and spasming. However, the main bonus for me was that I didn't get so anxious or stressed (even on the low dose), so my IBS improved overall.
Side effects I've had are a dry mouth and eyes (mainly in the few couple of weeks tho) and really vivid dreams - I now talk in my sleep sometimes, but it doesn't bother me!