I've ran a few KJW M4's and each one worked amazingly.
Internals were never switched, right now on my main KJW M4, I have an upgraded piston and velocity reducer. V2 bolt is on the to buy list, haven't had a single problem with the version one. The magazine design on the KJW's are genius (when comparing to the WE mags).
Out of the box, KJW is far better than the WE. *EDIT* In my opinion.
My brother runs a WE PDW, magazines are finicky and internals are just as susceptible to failure. Like people have stated, once you replace the internals with RA-Tech parts you're looking at just as good as KJW's internals.
I stand by my KJW M4, the only brand to replace KJW for a GBBR would be Inokatsu.
As for the magazine issues, all 8 of my KJW M4 magazines are perfect (have yet to replace anything) and I have had them for
about a year and a half. I even left my magazines in my gear bag empty for about 6 months without even touching them. Not one leaked.