Just installed one on my 500A. Fit is excellent. Rather than use the stock (included) A2 pistol grip I installed the Magpul MIAD grip (mainly to house a couple spare batteries for my Surefire forend). Initial impression - very nice upgrade. I will post a range review in the next couple of days as to any increase in felt recoil etc.
Just a note - the included pistol grip mounting screw was a bit long for the MIAD to mount - a quick trip to the hardware store for a 1/4 inch shorter (or perhaps a bit of filing on the inclded screw) will make short work of this problem - I went the hardware store route...not sure if this is an issue w/ other aftermarket grips, but just my warning if you are planning on something other than included grip. Threads are different than standard AR grip screw by the way.
Great product all in all...