I just own a KJW M4 GBBR but I really consider those guns for my next purchase
On what i found on internet here are my pro and con:
-possibility to modify to have the CQB version or long counter sniper version with the E
-The rails every where on RAS version
-The possibility to attach 2 mags together
-The mags system who cannot fit in every vest
-The mags system who is not compatible with Stanag
-Bullpup (the first one in GBBR) I love Bullpup!
-The sound when firing seems more real because the bolt is close to your ears
-It seems the kick is pretty much the same as the G39 (on review i read)
-The possibility to keep the mag for another WE stanag gun (M4, 416 etc)
-Lack of rails (side)
For me the Stanag is very important thing I consider since I will probably get another replica from WE with Stanag (HK 416) next.
In fact I dream of FA-MAS G2 GBBR with Stanag mag from WE but L85 is still cool.
We will find a way.