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.
I have a nice collection of manual focus lenses, and my camera has a built-in focus screen swapping feature. So I have the camera brand "precision focus screen". It is a more matte screen without split prisms, but I still get blackening noticeable enough to be annoyed when using a variable aperture lens. So I can only recommend something like that and something even more obstructive like split prism if your camera has interchangeable screens (for easy swapping) like mine and your standard lens is fixed aperture.