Bit late to the party, but I had a mate who bought a JG G3 with a rail system on it, really nice looking gun, very light because of the plastic body, but the performance was nothing to write home about.
Range was on par with most stock guns but the accuracy was pretty shocking, even on auto you'd struggle to hit anything beyond 40m, and the worst thing about it was that the grip was screwed into plastic, so if you ever put a lot of strain or weight on it, you're going to strip the threads and your grip is going to fall off...
Pretty major issue.
Then because you screw the grip into plastic... Well, the plastic you screw it into is obviously the main body of the gun, so you end up needing an entire new receiver set for it.
I don't think it's worth it, especially considering a G&G M4 will cost you less and perform significantly better, as well as being more or less the same weight, more modular, easier to upgrade and also, crucially, smaller.
Ease of moving a gun around counts for a lot more than you'd think.