Do a search on Rock River in the archives. They have an excellent reputation. I personally would be interested inknowing how they compare to a Bushmaster (probably as good as, but better?).
Anyone that's considering the purchase of a RR carbine might want to check into the AR15.COM special purchase. For $660 you get a pretty nice M4 configuration carbine with a limited edition serial number range. The ordering is being done through http://www.castle-arms.com/, all the specifications are there, check it out.
I do not own one but have heard nothing but praise for them. On a side note I have purchased some AR15 parts from them and customer service was top notch, I would do buisness with them again in a heart beat.
Ask your AR15 questions over at http://www.ar15.com. I'm sure you will get lots of help. I believe Rock River Arms has a reputation for making good stuff. I don't have any first hand experience with the brand though. They seem a little pricey compared to Bushmaster.