I have one of these 3rd party batteries for my K-7. While I did not experience any problems with the battery, my general observation is that it does not hold charges as long as the pentax original one (no scientific testing done of course) and field testing seem to suggest that the original battery can last longer for more photo shoots...but i think this is expected given that the 3rd party batt has less mAH capacity anyway.
Given the kind of price i get for the batt, i prolly shouldn't compain
But if i need another battery, i will prolly shell out more money this time round to get an original pentax one.