My two WE M4s have been pretty reliable and trouble-free. The NPAS (and understanding how NPAS works) is essential for reliable performance, not just keeping the FPS below field limits. I also feel they're more reliable/consistent with a heavier buffer installed but I don't have empirical data to support that (but offhand I think the extra inertia may allow a bit of extra gas to flow, net result similar to high flow valves).
Broke a bolt/nozzle on one once, but I know why it happened. Have not felt the need/been able to justify the cost to get the metal RAtech one.
By far the most troublesome aspect of them is the mags. V1s leak, I've seen a couple co2 mags develop leaks (the stock seal around the piercing pin valve is shitty) and I'm sure V2 mags will leak eventually.
All in all, I also picked up a WE AK and am getting rid of all AEGs save for LMGs. Both platforms have pros and cons and after 15 years with AEGs I'll gladly take GBBR cons over AEG cons. I carry a spare complete bolt carrier, so if something in that fails I can swap it in about 15 seconds; I carry a spare (stock) trigger group, if anything breaks I can swap that in about 2 minutes; if a mag leaks I can toss it and keep using the others. Other than that, short of getting run over by a car, my gun isn't going down -- no more getting sidelined by AEG stupidity.
Last edited by Drake; June 14th, 2013 at 10:52..