I did it two days in a row last year. Very dangerous, something I'd never want my kids to do, but biggest adrenaline rush I've ever had. There are two parts, the run down the streets into the stadium, then if you're far enough in front you can run into the bull ring. Once all the bulls get into the stadium they corral them all under the stadium then let them come back out one at a time. It's like the last hurrah for the bulls, people try to touch the bull and of course the bull gets some of them and really flips them around. It's absolutely nuts. I am a huge man, have been in many sketchy situations, this is one of the only times I actually felt unsafe and for a few moments feared for my life. But that's what makes life worth living, right?
Overall, the San Fermin Festival is an amazing experience, I had a great time. Drank more wine that any other time in my life. They mix it with cocacola, sounds disgusting, but with a lot of ice it is perfect for a hot summer day drinking session. I would recommend that trip to anybody who likes to drink a lot, meet people from around the world, and take in a little culture in Spain.