The "nuff said" when it comes to JG:
JG mags and batteries are complete garbage in my experience. The guns themselves are decent with fairly few lemons but rarely have I had their included hicap mags work well if at all. The batteries either wont take a full charge and have low voltage or will take far more than the rated mAh, put out a really high voltage for the cell count, and then totally die within a dozen cycles. I know others have different experiences but that alone shows the low quality control, the fact that sometimes it works great and sometimes it doesn't work out of the box.
JG is also the only brand I have bought that had a dead gun out of the box. This was when JG first came on the market though and their quality control has improved since then, quite a bit actually, but apparently not with mags or batteries. Also MP5 long hicaps in general are known to be temperamental, the double shortys are far more reliable.
Anyone saying they will buy a JG or ECHO1 I still recommend to save up extra and get a CA or G&P. Neither of those brands are perfect either but have MUCH better track records.