I had it done with my first and it was really painful for me. My doctor said if it was going to work it would be within 48 hours, but generally within 24. Well, the next morning my membranes ruptured. This might have happened anyways, but I seem to recall that it can cause it. They put me on pitocin pretty much right away (wish I would have demanded they waited a few hours at least) which made it pretty painful. I was starting to have contractions on my own, but I didn't really know it. I just felt crampy.
My sister in law also had it done and she got chorioamnionitis which can be caused by vaginal exams. This lead her to have a emergency c-section. She might have ended up with one anyways, but that didn't help with the matter. Not trying to scare you away from it, but do keep it in mind that it could cause infection. No idea on how common it is. Probably not very.