We recently purchased a A710 and we are very very impressed with it.
We bought it because you can hack the DIGIC II processor and get all kinds of crazy value added stuff:
You can shoot RAW (huge plus and a bunch of hours on the learning curve)
A live histogram
The ability to run scripts - for time lapse photography, dof "stacking", exposure bracketing, etc.
Heck, you can even use your lcd as an autoscrolling text reader and load a book and read it on vacation
I am NOT making this stuff up! It's perfectly safe too. They are starting to port the hack to the DIGIC III family but there does seem to be some problems specifically with the a720.
Not sure if any of the value added stuff is important to you but for us, the ability to shoot RAW in a sub $200 CANON camera with 7+ mpixels, 6x optical and IS? No-brainer! Add in a sub $150 CANON housing and, in our opinion, GREAT DEAL.
I specifically am not including a link but google "chdk wiki" for all the info you need.
Peace
TeerLkay
Pics from our first underwater excursion with the A710



Video from our first underwater excursion with the a710 (the very last clip on the video is with an a510-you can see the quality difference)
YouTube - Eagle Rays