I'm considering getting a microprism focusing screen for my Nikon d3000 (specifically, the KatzEye). Hopefully this will help tremendously with manual focusing, but I suspect there must be some disadvantages. What are the bigger issues with using a focusing screen?
here's a couple of potential disadvantages:
blackout: Split-prism focusing screens tend to turn black in the center with slower lenses (usually f/5.6 or slower, depends on the screen). Unless you're using a slow lens, or setting the aperture manually, this probably won't be a major problem.
effects on metering: Focusing screens can affect the camera's metering system, especially in spot metering mode. I haven't found this to be a major problem when I've used focusing screens in the past, but it depends on the camera/screen combination. Check the katz-eye website for your specific camera, they list the effects of their screen on metering.
visual distraction: The focusing screen breaks up the image in a way that can make it harder to see the final image.
cost: The katzeye focusing screens are expensive.
other issues: There are some other possible issues with split prism focusing screens, like a darker viewfinder and alignment issues, but I don't think they're as likely to be a problem with a katzeye screen.
I've used a couple split prism focusing screens on different cameras and have liked them, but I haven't used a katzeye.
I have a Katz Eye in my Nikon D200, and while I love it, the issues raised by @walter in his answer are there. Visual clutter is the thing I notice most.
Another potential issue is the risk of damaging your camera. Rooting around in your expensive piece of photographic equipment can a hairy experience.
That said, I consider myself being average when it comes to the do-it-yourself department, and I had no problem changing the screen myself.