I'm not a huge fan of quad-rails but I'm currently using FireField aluminum handguards on my Draco SBR because I wanted some mounting options for something different than my wood-and-steel guns, and the Ultimak quad rail wasn't going to happen at $300 for the set and the MI rail wasn't available at the time... The Firefield works well for my PA Mico and a VFG, which is all I need it to do. The Firefield isn't much different from the UTG quad rail in price or design, but the Firefield is noticeably lighter (which is why I chose it) and had a better finish. It heats up pretty bad after a couple mags, but with a VFG that doesn't really matter. The sight holds zero reliably while mounted on there so long as the handguards aren't removed, and does not co-witness (which I don't care for anyways). There are certainly much better options, but for a cheap "experimental" rail at less than half the cost of the MI, I think it's great. I had a coupon for the place I bought it from so my cost was like $40 or $50 shipped.
I don't have any experience with the others posted above, so I just thought I'd throw my $.02 in here on the Firefield.