I had one, and it worked quite well for me.
My problem was that it juts out farther than the regular eyepiece. I'm fine with that, but my camera bag was not, so one day putting my camera away the edge of my camera bag snagged the eyepiece and popped it off into the snow. I didn't notice until the next time I went to take a picture, about 5 minute walk away. I returned to my previous location but failed to locate it.
Next time I have money to burn I'll be buying a new one. I must say though, when I end up buying a second DSLR (to have a backup) I'll be buying one of the Pentax models that have the better viewfinder to begin with.