I have recently upgrade from the A700 to a s/h A900 - having been seduced by one when a friend lent me his for the day last year.
Usage wise, everything is exactly where you expect it to be, switching from one to the other is very easy.
The viewfinder is bigger and brighter, but the difference is less than between the A200 and A700 (IE the A200 VF seems small when you are used to the A700, the A700 seems smaller, but still good, if that makes sense!).
The big difference is the images.
The added dynamic range of the A900 is very obvious once you have used it for a days shooting - images need much less pp to recover highlights and shadows.
The larger sensor also means that you have more scope to crop for composition if needed. It actually has a very similar pixel density to the A700,
so if you crop heavily in pp you will get the same results as you would from an A700 - IE Take a pair of shots with the A900 & A700 using the same lens (and focal length, if a zoom). Crop down to 800x600 from both, and the images will be very similar in terms of FOV, etc. So going FF in this way looses nothing in terms of 'reach', but gains in terms of wider FOV when that is what you want.
Only problem is you find an irresistible urge to buy FF lenses to match the A900's quality