I have to admit, I love participating in studies. There is always a good reason to do so. In one study, I was on a fantastic insulin for a year -- I can't wait for that one to get FDA approval and become widely available! In another study, I was in the control group, so even though I didn't have to change anything, I still got paid for my time and had my medicines provided to me.
I've also done some short-term studies, some as short as a single day or even just a questionaire. Some studies pay quite well, especially considering the amount of time you put into them, and of course having medications provided is almost like getting paid, too -- as any diabetic knows, you can spend a lot of money each month on insulin, needles, test strips, and so on. Getting that stuff for free almost always makes the studies worth doing.
But most of all I love learning more about diabetes and the newest medicines and technology for treating it. There is often an opportunity to learn more about my body, too. For example, one study I did took ultrasound readings of my heart, which was kind of cool to see. Another did several nerve conduction tests, and it was interesting (not to mention quite a relief) to learn that I haven't lost any nerve conduction in my lower legs and feet since I've been diabetic.
There are lots of reasons why it might be beneficial for you to do a medical study, and the check you get at the end of it is only one reason out of many!