Some things I like about the program include how well-integrated the two degrees are, and the community of support built for MSTP students. Like most MSTPs, you meet with the other MD/PhD students frequently during the med years for research presentations and such. Then, before going into the grad program full time, you do at least one clinical rotation. This opens up the opportunity to take part in the Longitudial Clinical Clerkship while you're doing your grad work. This involves spending a half-day a week in the clinic with a doc - just you and him/her, so you can learn more than in a regular clinical rotation. What's great is that you actually get credit for this clerkship - every 20 weeks you are given credit for 2 weeks of clinical rotations that you don't have to do after your thesis. Also, when you do return to the wards full-time, you haven't lost your clinical skills.
Also, Clayton the director and the administration seem very supportive and influencial in making sure students finish their projects in a timely manner. If your PI is keeping you too long, you aren't on your own - the program will attempt to help move things along for you.
Doing a research rotation before first year med is optional - most students don't do it. The average time to graduation is 7.5 years, and has supposedly been growing shorter (according to Clayton).
Anyway, it seems like a great program and a cool place!