Personally...I've owned several KJW pistols that have just worked. Shot after shot for lots of shots.
None were spectacular or very refined in their fit and finish...but they shot. I wouldn't start with one to build a custom gun, but I wouldn't hesitate to game one and beat it about. Aside from the large gas consumption and so-so mags, they're just fine.
I've been known to fiddle with things on occasion
...and they didn't lend themselves well to modding.