I've tried the Pentax, Nikon and KPS eyepieces and I use the KPS one. Beats the others
More magnification, great optics and VERY well-built. I love the way it looks too, low profile and looks like the D3 eyepiece.
If you wear glasses though you'll have a harder time since it doesn't have a very high "eyepoint", your eye has to be close to the eyepiece. It does have a feeling of "vignetting" but I don't mind it at all. I still see all the info and after a week it's gonna be normal.
Use it on my K20D and it's now about the size of the 5D's viewfinder. BIG difference for me since I'm spoiled with the MX/ME Super's viewfinder
Got it locally for about an equivalent of $50 USD and I love it. It's kinda hard going back to smaller viewfinders