I had this done with my second child because I didn't want to be induced (which they had scheduled for the next morning) and they thought I was having a big baby (they were right there, he was 11 lb 1.4 oz). I had my membranes stripped around 5:30pm and was in the hospital by 1-2am in active labor. I must say though I had a lot of prelabor the night before, and I think my son was coming in the next day or two anyway.
It's a very quick procedure but does hurt like the dickens when they do it. I guess you could sort of compare it to removing a band-aid from a hairy portion of your skin. You yelp cause it hurt so much, but then it's over and their is no more pain.
Good luck!
K. (mother of three, two boys and a girl, audiologist, doula, crafter, minor event planner...)