Got the M screen today finally. I wish I had read the above article provided by David before installing it, as I had trouble putting the screen in place, expecting it to fit inside the frame. In fact, it doesn't go into but on the frame. Apart from that, everything went smooth, especially that I have previously changed screens many times on my Canon 1Ds cameras.
The M screen is visibly darker than the standard G type, but manual focusing is much more accurate (and easier) as advertized. I measured 1/3 stop difference between the menu settings, so I decided to keep it on G since a +1/3 is quite welcome since I usually would dial in at least that much when I'm shooting in normal lighting.
I did a few handheld tests with the 50/1.4 at 1.4 and realized manual focusing is sharper than AF, probably because this lens has a slight front or back focus. I won't bother correcting that.
Anyway, I would strongly recommend this screen to all forum members, since it makes manual focusing so much easier, and does not affect AF in any way for those times when AF is needed.