I think you're better off getting a focusing screen, provided you're confident in your ability to install it. I've heard a couple of horror stories about the AF-confirm adapters totally destroying a camera's circuitry...for some reason, most often on 20d's. I think this is a very rare occurence, but it's still something to think about.[/QUOTE]
I have never heard of the AF confirm adapters harming a camera, but why take a chance? Also, I have heard from a number of sources the accuracy is none to precise. And let's face, many times we go with MF so we can use our large aperture (f/1.4, f/1.2, f/1.8) lenses - exactly the one's which require the most precise focus!
I use the slightly courser "S" screen on my 40D and an Olympus ME-1 1.2X magnifying eyecup (yes it fits Canon like a glove). Together this combo works very well for me. I can see even slower lens "snap" into focus. You lose a little brightness with the S screen, but it is a Canon product made for the 40D, so no danger, and very easy installation.