We didn't have any, but our dinner service was pretty fast (we did a combo first course instead of separate salad and pasta courses, so we had one less course than usual). I think it's cool when the performance is something important to a couple - say, traditional dancing at a Greek wedding or something similar - but I have also seen some really cheesy lounge-singer-type stuff that hasn't worked AT. ALL. Choose your performer carefully if you go this route, and don't feel like you have to do it if it's not in the budget.
Re: dinner entertainment