Re: CMMG MK-8
I helped a friend get one up an running and it was not a bad setup. Like Match308 said, you can get aluminum G3/HK91 mags for next to nothing ($1/mag or less), but the CMMG lowers that use the G3 mags are only compatible with DPMS uppers...and even then, you have to modify your DPMS upper by removing some material from the walls of the upper to allow the mags to fit properly (the G3/HK91 mags are wider at the mouth than the DPMS/Armalite mags). Also, you lose the LRBHO function that you get with other mags/systems. CMMG and outside sources are working on the LRBHO issue I believe.
Aside from those things, they are decent setups, reliable and very cost effective given the cost of the mags at 1/20 or so the cost of DPMS-pattern mags.