JG batteries aren't the best, but they aren't horrible either. They will power your gun nicely and last for a whole day's gaming without problem. Whatever the mAh rating, you can expect to get at minimum 1 shots for every mAh.
However, their life expectancy is shorter than other batteries. I would STONGLY recommend you also order a good charger - one with overcharge protection. The wall-wart chargers that come with these guns should ideally be thrown right in the garbage as soon as the box is opened.