I know the 1DsIII and the 50D can do this, but it's new to me with the 5DII.
I can frame a shot, zoom in on the LCD to wherever I can focus, focus, and immediately take the shot. I don't even need to zoom out if the subject is unchanging. I have done this both handheld and on a tripod. Yes, I've become one of those people holding the camera out in front of me taking a shot (if I can set a high enough shutter speed).
Even with exposure confirmation adapter I sometime have trouble getting the confirmation beep in low light.
This isn't news to many people. But if you don't have a 5DII, 50D or 1DsIII don't underestimate the focus aides compared to older cameras. Today I've used it both on a Biotar 75 and a Nikon 14-24.
