As others have mentioned, it depends on the program. At my program, you have the option of doing a research rotation the summer prior. I chose to do so, and I regret it. My time would have been much better spent doing something fun unrelated to research. You're expected to do something work-related pretty much every summer for the rest of your life. This is seriously the last summer vacation.
While you do get to move to a new city and set down roots, there will not be that many people around so your free time will be somewhat boring.
With regards to whether or not the MD/PhD is worth it, definitely read the thread K31 posted as well as the "How to Fix the MSTP." It's full of many somewhat jaded opinions that no one else is going to tell you, but I think contain a tremendous amount of truth.