We participated last summer. Being an east coast startup, it gave us the chance to get a great feel for startup life in the Valley, prove out our model, and build a beta version of our iPhone app which we launch in November (www.bit.ly/IHeartPushpins). Most importantly though, I learned more from doing the program than any summer internship I ever had.
If you want an honest review of the experience, shoot me an email jason@pushpinsapp.com.