Haven't had any problems with my DPMS Extreme (24" Super Bull, Hi-Rider). Muzzle heavy, of course. I put in a 3# lead weight in the buttstock to help balance it out. The gun is not for carry. The handguard on mine is increadibly tight (to unscrew). When I bought it, I had forgotten to specify for a sling swivel stud on the hand guard so I had to drill my own. The handguard is aluminum so it wasn't difficult to do. The barrel inder the handguard is not finished like the portion seen in front of the gas port. Not a biggy though as it is not seen.
$559 sounds like a very good price, particularly if with the bolt assembly. If not, it's still a good price if new or nearly new.