My brother used to have one.
It was.... different.
I didn't like it, but it shot really well. It never had a bobble in the 3 or 400 hundred rounds we fired through it and my brother bought it used.
I didn't like the way the magazines went in or how they had to be pulled out.
I hated the rear BUIS, I think I would have done better using the front sight like a bead sighted shotgun.
I did like the lightness and how short it felt.
I thought it was very comfortable to shoot.
I thought the brass management was a really cool idea.
I liked the way the former owner had added a flashlight into the front handguard. (6P if I remember right)
There is a guy on M4Carbine.net that ran one in a Pat Rogers EAG course and wrote and article for SWAT on it and it worked well. (EdL is his screen name)