I think this is a wonderful (and cheap at the price!) mission.

http://www.spacedaily.com/news/grace-02a.html

Further, it makes me wonder about an interesting Mars mission concept. Start with GRACE, which can tell a lot about what lies beneath the surface. And since you must have two satellites in close formation anyway, add a radar receiver on each for radio wave interferometry. With the separation of the GRACE platforms, you could get awesome resolution. Then add a radio emitter in the wavelengths chosen, and an upgraded communications package. Could do atmosphere occultation through the limb to earth with the emitter, too.

The only thing about GRACE that would be less beneficial at MARS is that the gravity map of Mars should not change much over time. There aren't huge amounts of liquids and gasses moving from here to there, like on Earth. Though it would be amazing to be surprised in this regard, I doubt it would happen.