Slow charging is better for a battery's health - but I wouldn't worry too much about it (especially 850 vs 1000, not much difference there, 500/1000 maybe). Also these are big batteries they can take the higher current.
I think these batteries are good for 500 charge cycles, at which point they start to not perform as well, but even in this state they usually remain very reliable. I've had a li-ion battery for four years that seems as good as the day I bought it.
These batteries work with chemistry though - so there are always environmental differences between users and people do different things with their phones, I think this accounts for some people saying battery life is great, when others say bad.
