AF wise it wouldn't be an upgrade or downgrade, just the center point is accurate, pretty sure the performance between the two are the same. Plenty of people don't like to limit themselves to just the center AF point, with the 5DmkII it isn't lightning fast so tracking moving subjects can be tricky and in low enough light it may not nail focus.
Then again, should be the same experience with the 450D, for still portraits, landscapes and general purpose it's fine. The 6D is similar with only having an accurate center AF point, but apparently it's much more sensitive and accurate then that of the MkII, so if you're considering then you could wait. Which, for the reviews as well, would be well into late January as I see Amazon Japan telling me the camera is set to come out right before the first day of 2013