I got one for my D90, and found it more ho-hum than other posters. It's not cheap, either. There are a couple of vendors in HK, I think, that sell screens for $20 or so, but I have no idea if they're worth $20 or anything close to what Katzeye charges.
As for performance, it's not equal to a K screen on an F3, but at least there is the split image and the micro prism donut. OK, I just compared the D90 with my non HP F3, both with zooms allowing a more or less equivalent 50mm fov. The F3 has higher magnification than the D90, so the easier viewing is not the screen; it's the camera.
Tip: Make sure that when you use the screen for manual focusing, that you move the active focus area out of the central circle. If you don't, it'll be difficult to see the focusing aids you paid for.
I got my screen just before I had to visit Gerry Smith about a Leica lens, so I took the D90, the installation instructions, and the screen with me. He very obligingly installed it for me, and didn't charge me anything. I was sure I'd take hours to do it, and would scratch the screen. I am a Gerry Smith fan, both for the work he does and for his work ethic. Oh yeah, he took about three minutes to install the screen.