I didnt use knee pads when i first got mine - i got bruises from the kneebar everytime i wore them, and they dug in after about 20mins, making it difficult to bock for long periods of time.
Then I started using the Pit Dirt ones from here - no more bruises, in fact they are fairly well padded so are actually more comfortable. I also loop the elastic straps over the back of the bock frame making my leg much more secure.
Id definitely recommend these ones and would never go back to bocking without kneepads!