I own a stock Echo 1 M4A1 (Stag-15) which I've had for a good few months now and used in a few battles, and it's worked great. Solid build, accurate, good ROF even with the stock 8.4v battery, decent trades, and good FPS. I've been very satisfied with my purchase and would recommend Echo 1 airsoft guns to anyone.
Only thing I would change is I would've gotten the RIS variant instead of the classic carbine, as you get amazing quality full metal rails and a bit better battery in the PEQ box for only $20 more.