I have one on my K-7 and it is great - crystal clear and does enhance the view area and helps with manual focus.
I also take my glasses off and although my diopter adjustment is not at the limit it is still very helpful.
I think it has been covered elsewhere but given the pronounced profile it helps avoiding getting imprints from your nose all over the lcd screen.
I think that the price is well worth the investment - although this is only IMHO
I can only say that my only negative is that my other cameras do not have one and I am always forgetting this and get frustrated by the small view area before I remember why
Cheers