So I just received my A550 and have had some time to play around with it enough to get some of my first impressions.

The Good:

*Faster AF

*Much faster FPS - 7 FPS is insanely fast compared to the A300.

*Easier to use menu system

*Better button layout

*Excellent LCD screen

*Better on camera flash (granted I will not use it a lot)

*Solid build (similar to the A300)

*Better viewfinder

*Comfortable to hold (A300 is also)

*Sharper images

The Not So Good:
*The position of the power switch is just dumb. Three times I turned the camera off when I thought I was moving the jog dial. I will get used to it though.

*Annoying Red light somehow bleeding into the VF when focus is locked.

*Two tone color is ok, but would prefer all black

Those are quick first impressions. Overall, I am very happy with the camera and it is a definite upgrade from the A300. Lacking Program Shift, Mirror Lock Up and a few other widely discussed features on this board do not bother me too much right now. As my experience grows, I may find myself wanting more features, but for now the A550 has all I need.