I put an M screen in my new A900 just the other day. I wanted to play with a bunch of M42 lenses, and I found the standard screen to be pretty hopeless - its brightness is definitely achieved at the expense of focusing acuity.
Some observations after a few days of snapshooting with the M screen:
* The AF marks are still there.
* Focusing acuity is distinctly better, but the screen won't solve all focusing problems by itself. Focusing a 35mm f/3.5 on the go is still a challenge, even under bright light; 35mm f/2.8 is easier, 35mm f/2 is easier still, and 50mm f/1.4 is quite nice. (I haven't tried anything longer than 50 or shorter than 35 so far.) Where critical focus matters, I still have to put the camera on a tripod and use the Sony viewfinder focus magnifier to take full advantage of the extra acuity. Of course, my judgments here are colored by the limitations of my own eyesight.
* To state the obvious, to get the most out of it make sure you have your finder diopter setting tuned.
* It's much dimmer than the standard screen. It's very dark with a 35mm f/3.5 indoors under my usual moderately dim interior lighting. Still fine for composing, I think, but many may disagree, and I can see why Sony recommends nothing slower than f/2.8. Outdoors in bright daylight, f/3.5 is fine.
I expect that I will leave the M screen in, even if I end up using mostly AF lenses. Decent focusing acuity is much more important to me than extreme screen brightness.