I have a friend, willing to offload a Piston-driven STAG-15 with a bunch of mags and a VERY low round count. I don't know much about STAG Arms and would appreciate some input.
Stags are OK. The quality of the parts is good (Stag is the retail side of Continental Machine & Tool, which makes parts for a wide variety of AR assemblers/manufacturers), and I haven't heard of any problems with the complete rifles I've sold at Bradis. I believe the Stag Model 8 piston gun uses a 3rd-party piston kit (likely Adams Arms or PWS), much like the S&W M&P-15PSX.
My first and only AR (so far) is a Stag that I put together. The looks and quality are there but it doesn't like to eat much laquered steel case ammo before it wants cleaned.
i have one, haven't gotten a chance to make it to a range that allows me to shoot 5.56, but everything about it seems nice, and they come with a lifetime warranty. couldn't really ask for much more, especially at the price.
Stag is good to go. As others have said, they are one of the largest producers of AR's and AR parts in the world and actually make the parts for many popular brands such as the S&W M&P series AR's. When I talked to Stag about their Model 8, they told me that the piston system is a design they came up with in house after looking at what else was out there. From the pics I have seen, it looks pretty close to the Adam's Arms kit. As long as you do not mind the deer head logo, buy it.