@Richard: Are you sure the "last AF is on the shutter" ? Because if I aim at something white and the AF-light strobes and the AF just sits there and I press the shutter, it does NOTHING, but if I release the AF-ON in the same no-af possible situation, it tries again. Same thing in One Shot, if I lock with either button the other doesn't do anything as long as the first is pressed. SO if the AF-ON has priority then I can keep AF on both buttons for reasons mentioned below.
Ah, okay... So I set the shutterbutton to meter when pressed.. Yeeeeah, well then I guess it might not be for me then, just because, If I want to shoot a shot in One shot, I first have to press the m...f..n button to switch to One Shot from Ai, then push the AF-ON button on the back, hold them and then push the shutter. To me, using three buttons to take a picture is ridicolous.
In Ai though, it's pretty great! Canon should do a combo, where I can have One shot at the shutter button alone, and then Ai on the AF-ON button, so when it's pushed it's Ai on the AF-on and only release on the shutter, and if I just want one shot release the AF-ON and take shot like you always have (with One Shot focus).