I asked this a long time ago and got no replies. Just wundering if they break like other plastic lowers?
I'm talking about the newest ones that come with a jig and all the bits.
At this point I've got enough ARs just messing around, thinking about getting into my first plastic lower.

I bought one just for shits and giggles. Very easy to finish out and the only thing that gave me a problem was the hammer and trigger pins tend to "walk" out. I installed anti-walk pins and cured the problem.
I wouldn't trust my life on it but then again I'll never have to. Plenty of others for that.

I've got several thousand rounds through polymer ARs using EP Armory lowers (the brand sold by Ares), and have yet to have a problem with any of them. They aren't top-end WRT fit and finish, and may require deburring and mold parting line cleanup. But you can build a decent reliable AR from them.

They are easy to mill out. Even after you clean burrs out of the detent holes and sand off parting lines, you'll probably still be faster than milling an aluminum lower.

Also, they are noticeably lighter than aluminum lowers. Something to think about if you want to build a lightweight gun.

I too have over 2000 rounds through my Ares Polymer80 lower and am very pleased with it. It was incredibly easy to mill with a standard drill press and dremel tool. And paired with an Ares upper it shoots incredibly well.
http://i62.tinypic.com/34snus7.jpg