Check this system out:
http://www.youtube.com/watch?v=8W_nJGyoKio
http://www.youtube.com/watch?v=b76V0j15zfo
Of course blowback is not necessary, but some like the added dimension. This allows you to convert your plain jane M4 to the world's largest vibrator

All kidding aside, this looks pretty cool IMO. The only concerns I would have is:
-Possible added internal wear due to the system using the piston
-FPS loss
-No way to use a buffer/crane stock battery
So, if you have a front wired M4 (or willing to convert) and are not scared with opening a gearbox, this thing should be right up your alley.
Available now at WGC Shop.
P.S. They recommend not to go above 18 rps with this thing.