Yes, my husband and I have both done it.
They do an initial basic health test (height, weight, blood pressure, etc), then track you over the next few years (I believe). We haven't been called back for further tests yet, but I understand they are doing research into all sorts of medical conditions, and who develops things in the future.
1000's of people are already signed up, but they need lots of people to create enough data to use. (Different ages, locations, familial histories, pre-existing conditions, etc.)
In our family we have an inheritable condition, which sadly our daughter was born with (our 2 sons luckily missed it).
I will do anything to help research into this and other medical conditions so that treatments and cures can be developed, and other families do not have to suffer what we have in the future.
Do get involved if you can, the more information and people they have the better...