The X-Grip I ordered from Top Gun arrived last night. It really makes a difference in the feel of the M&P compact with the full size grip; I'll probably carry it with the full-size magazine most of the time, unless I really need to conceal, then I'll use the flat floorplate mags, not the finger extension.
The grip is ultra-easy to install, just slip it down over the magazine; no latches, screws, or anything to hold it to the magazine. It is slightly loose when the magazine is not in the gun, once locked in place, the grip is stable.
Good piece of equipment for $12.95.