Originally posted by Pauld
In terms of functionality there is no difference but in aesthetics there is a big difference. I purchased an OEM grip before finding a good deal on a genuine Pentax grip and there is a big difference. The OEM grip felt like it was made of thin cheap plastic, and the textured part of the grip was quite slippery. The shutter button also wasn't as soft as the genuine grips button and at times a little stiff to press down to get auto focus to work.
I also own both at the moment and echo the above sentiment. Yes they are visually the same, but hold them and touch them side by side and they are obviously made from different materials, and the pentax grip is sealed so buttons are more responsive and everything is to a tighter build tolerance.