I've got an after market focusing screen.
Specifically this one "Rule of Thirds" split image microprism focusing screen for PENTAX K-7. .

Who else uses alternate screens?

I like the split prism for manual focus!
My 1977 vintage MX has one and I have one on my K20D as well.

FYI There are three main alternates that I've found:
a) Katz-eye
b) Focusing Screens
c) Other eBay cheapies

So what are your experiences?