My school requires them! I'm a first yr so I can't comment on exactly what is beneficial, but I do know that it's pretty diff from doing rotations at a university hospital. You just see different things and do different treatments. I think they act as an all purpose doctor because there may not be nearby specialists so there are a lot of things you can do. You also get a diff perspective on medicine. I spent a week with a physician in a rural area and he was basically giving handouts. People hardly paid for the visits and didn't have money for prescriptions so he would have to give them a ton of samples. All people in rural areas aren't like that of course, but it just so happened that this clinic was the place those without insurance or money went to. Gives you a very different perspective on medicine in general and you see how much these people need young competent doctors that are up with the new technology.
Like most things - it depends on the clinical supervisor, the presence of other students or residents and your motivation level.
My rural surgery rotation was fantastic - we did everything from Gen Surg to Ortho to Plastics and because they were understaffed and overworked after awhile, I would do operations (little things like I&D'ing abscesses) by myself without an attending or resident scrubbed.
IMHO they are fantastic but ask around about sites which have proven beneficial to others before you...