I own a FA91. It is an American made 'clone' of the HK91. Instead of a steel receiver, it uses an aluminum one.
I purchased mine for $450 including 4 magazines and a sling. I would say that $600 is quite a bit too high. I believe I saw them brand new for $400 in FA's last catalogue.
Mine has been reliable and accurate. The built in rail on top of the receiver is great; you don't need to spend $300-$400 more for the H&K rail to put a scope on it. Most of the internals on my rifle has an H&K stamp.
The downside of these rifles is that they are not the same dimensions around the mag-well area as the original welded stamped steel receivers. Therefore, it is impossible to put on a wide forearm unless you really cut up the forearm. You'll be stuck with the thin forearm, and it heats up kind of fast. This doesn't bother me as I wear gloves when shooting.
If you have any further questions, feel free to e-mail me.
Unfortunately, I don't have any really good pictures of the rifle online at the moment. Here's one that is a relatively good view of the receiver.
http://moon.pr.erau.edu/~thompsj/firearm/images/picture/lowready.jpg