There's my JG MP5 review from almost 3 years ago.
JG MP5-J Review
Pay close attention to post #49 on the second page. That describes what I did trying to kill this gun and couldn't.
Here we are a year and a half after Post #49, and amost 3 years after the initial review. The gun is still going strong on the stock JG internals, still shoots a consistent 345-355 fps on 0.2g, and is still incredibly accurate at ranges such a short barrel shouldn't be able to hit that accurately. Note the gearbox, spring guide, cutoff lever and gears were replaced, but everything else is stock in terms of the compression parts, and it's still one of the most awesome little AEGs I've ever had the pleasure of owning. I've gamed it myself in summer, winter, rain, etc, I've loaned it to others, and everyone who's used it absolutely loves it. Those internals are 3 years old, have seem probably close to 100k rounds, and are still going strong.
All that being said though, JG is still a Chinese brand. In the last couple of years, JG have made enormous strides ahead in quality control. But the possibility of a lemon is still there. Then again, you can easily get lemons in any gun at any price range, from a $250 JG all the way to a $2500 Systema. The advangate of the JG is that even if you have to replace a fair portion of the internals, you're still ahead financially and with better parts than buying a TM.