Although I normally respect and abide by Troy's advice, I have to dissent on this one....
Once the mount is on the barrel, it locks up nice and tight. I have had no issues of flimsiness or looseness on mine.
The only thing that should be of concern about these units is that you have to mount them in front of the front sight. This can be an uncomfortable reach for some (Hence the adjustability on the RIS/RAS units). On my rifle however, I modified my handguards and mounted the unit sight-unseen under the handguards, closer to the receiver. Very clean looking.
The raked-forward grip is basically no different than the Thompson submachine guns of days earlier; so if you don't mind the feel of those, you should have no problem with the BM one.
If your own personal ergonomics dictate that your forward wrist is perpendicular to the barrel, then maybe a more "conventional" grip is for you. But for the price, you can't beat the BM grip!