There are better dampening systems out there that give much more control without sacrificing the tone as much. However, Moongels have their use and I wouldn't go anywhere without some.
They do what they say on the tin, they dampen, and control the overtones, at the expense of some of the tone of the drum itself.
But they last, they can be washed to last longer, and if I loose a couple I won't cry about it. Sometimes, you need to go heavy handed with a few of these to control a bad kit with bad heads.
But if you are looking for something that will allow you to fine tune the sound you want, there are other (better) options