Been on it for....hrm. 3 months? 4? I was on Effexor before it. (Note: I'm also on Lithium) The way my psych explained it to me was that it's a lot like Effexor in how it works...it's just a metabolite of the same drug, so it absorbs quicker, and you can take a smaller dose.
I haven't noticed any difference in how I feel since switching over. It DOES irritate your stomach, so take it with food. Also, there's a card you can get from the manufacturer that will cut your copay in half.
PM me if you want more info or just want to chat about it.