A friend of mine did it a few years back....don't bring your "good" wheels, as he saw plenty of riders destory their nice, lightweight racing wheels. Also, if you're riding in a big pack of people, be prepared for anything.....this older woman who had no clue what she was doing and kept bumping into my buddy. At one point coming around some riders who fell, he ended up putting her over the curb and into a non-movable object (I think it was a tree or lightpost) to avoid hitting them.

I guess if you ride with a group who knows what they're doing, it's not too bad.....and if you ride fast enough, you should be in somewhat good company instead of the 20,000 people behind you who have never ridden in a pack before.