I've run with the bulls twice.
And here's the deal. It's scarier the second time because you know just how effed up it is.
You have no idea how fast those bulls are. And they are MASSIVE. Not only that, but Calle Estafeta (the long, straight road you run on before you enter the bullring) is maybe 10 meters wide at the most.
And there are probably 2,000 runners (most of which are either hung over or still drunk from the night before) all fending for themselves.
The rockets go off at 8:00AM... which means the bulls have been released. At 7:58AM, I literally pissed my pants I was so scared.
If you go, make sure you're on the course at about 6:30AM. Wear good running shoes. Do some stretching. Prepare for an all-out sprint. And I'd recommend going about 3/4 of the way up the course so you only have to run a few hundred meters or so before you enter the bullring.
You'll see the bulls around 8:02AM. And when you see them -- trust me -- there's a damn good chance you're gonna brown your boxers. I don't care how tough you are. It is absolutely terrifying.
What do you do when you finally see the bulls? RUN! RUN FOR YOUR LIFE! And make sure not to fall down because you'll get trampled by other runners (or you might get gored by one of the bulls. I even saw an American guy get paralyzed (he's better now - a medical miracle, apparently) in 2006.
Just run, run, run and once you're in the bullring, it's not over... because that's when they let out the baby bulls and they are effin mental.
Enjoy!