Well I've seen some people inquire about this issue on this forum but haven't seen anyone with personal experience comment. I have just recently acquired some personal experience so I thought I'd share.
Some background. If you haven't read any threads on this, the basic idea is to replace the focusing screen in a DSLR (which is designed for an autofocusing system) with a focusing screen that makes manual focusing easier. Currently, I have to judge the focus based on what my eye can perceive in the viewfinder or from the focus indicator located down in the bottom left corner of the screen. I plan on picking up some cheap (in price only) old manual focus lenses in the future so I would like my manual focusing to be as efficient as possible. Traditional focusing screens with split prisms are much better suited to this.
There are several places where you can get focusing screens including Katzeye, Hoada, Virtual Village and ebay. They run from $100 to $20 on ebay. While $20 sounded dangerously cheap, I had the name of a particular seller who had provided someone else with a satisfactory product, so I went cheap. The other brands will offer more options for the screen layout and provide more accesories to make installation easier. The ebay option will also take about 3 weeks to ship because it's coming from China.
Once you've recieved your screen you get to install it yourself or pay a shop ~$60 to do it for you. Since the whole point of this venture for me was to be as cheap as possible, I did it myself. Honestly, it wasn't that bad. I thought I would have to take apart the plastic housing and lose little screws in the cracks of my floors but it was much simpler. Here's a youtube video that runs through the motions.
http://www.youtube.com/watch?v=3IjSPL584zg
The hard part is keeping it all clean. I was wearing latex gloves, had pec cloth handy, and was swabing everything insight and still ended up with a messy viewfinder when I was done (mostly dust specs). Note that this doesn't effect the actual image, it's just really annoying. I'm planning on going back for a good cleaning once I find a cleaner environment.
The other tricky part is getting your manual focus calibrated. My replacement screen was 1-2 THOUSANDTHS of an inch thinner, which made it notably back focus (the camera focused behind the intended spot). If you're really far off, you'll have to add shims to move the screen closer to the mirror. Luckily , I was close enough that I could make the necessary adjustments with the calibration screw in the mirror box. If this is all starting to get confusing, have a look at this link. It helped me understand all of the focusing systems in my camera. It's really very interesting regardless of whether you're doing this installation or not.
http://leongoodman.tripod.com/d70focuspart3.html
One thing you may be wondering is how does all of this messing around inside my camera affect with the auto-focusing system. Well, as far as I understand, not at all. The auto-focusing sensors are located in a separate part of the camera and are not affected by the new focusing screen or he adjustment of the calibration screw (if you turned the right screw).
After all that, I have to say that the screen is wonderful. Focusing is miles better with the split prism/microprisms. Best $20 I ever spent on my camera. I can't wait to pick up some old fast primes to try out now.