I went through it a few months back, it really sucked! I had my nose broken(Didn't realize it till later that night when my wife told me my nose was crooked), one guy had a tooth knocked out and two guys were K.O.'d(one guy twice). Other than that it wasn't too bad. The most suckiest part about it was you can't hit back. I'm not used to being punched in the face and not hitting back.
If you haven't gone through it yet the best advice I can give is, get in quick stay low and keep your guard up. Once you get a hold of the instuctor hold on for dear life and get in there for the clinch. I got lucky the forth time through, which is supposed to be the hardest,I outweighed the instructor by about 80'lbs, but the first three times the instructors were about my size and a lot harder. Good luck!