Well, I've had the A900 for a couple of months now, and over Easter got to actually use it while 'out and about' with family.
In use it is very similar to my A700 - but bigger (heavier!) and brighter.
However, it's on getting back home and importing the images to LR that I really noticed the difference.
I am getting a noticeably higher 'success' rate - so more of the pictures I take are potential keepers, and they need less adjustment in LR to bring out the best (IE Less Fill or Highlight Recovery).
The two may, of course, be related (since they look better before adjustment, they are more likely to be considered as keepers).
The only downside is that I need a bigger bag