I have a dpms m4, and after taking the whole thing apart, i have to say the quality is actually alot better then some of the other guns ive seen. They come with steel bushings, shim was decent from factory, and so far has been holding up just fine.
The gun is completly TM compatible, and will take to upgrades just as well as any other guns. I put an upgraded spring, piston, bearing spring guide and a tight bore on the gun and i absolutly love it. Only thing that was lacking right out of the box, was the battery. I have a lipo here waiting to go in as soon as my charger gets here. We'll see how the gun likes my M120 spring thats in it and an 11.1V lipo