I've had my M15A4 UAC for several months now. The externals are fantastic (working bolt catch anyone?), and the internals are great as well. As of yet, I don't have any gripes about it, and I haven't had any problems with, either internal or external. Pretty sweet looking trademarks as well.
Mine is pretty much stock, I've put in a Madbull tightbore and an H nub. I'm using an Intellect 9.6v 3600mAh battery in it. For a virtually stock gun, the ROF is great (haven't tested that officially though). FPS is right at 395-400.
They're a little on the pricey side, but if you have the money, totally worth it.
