I tried my hand at manual focus yesterday. I put my dad's 50mm f/1.4 lens on my D40x and figured I'd play with it and see how it went.
Verdict: People with poor eyesight should not use manual focus.
In general I trust the camera's focusing over my own eyes, so manual focus was tough. Also, I kept the aperture down to 1.4 most of the time, so it was really hard to find my focus area. I found that if I set the focus ring all the way to...I don't know what it's called, but it's the not-infinity end, where you're focusing at the closest range. So I set the ring there, and then moved my head forward and back until my subject was in focus...or until something was in focus.
And the when I pulled it all up in Photoshop this morning..I'm surprised by what was in or out of focus.