I had the 2nd gen time capsule in my apartment, and I had an airport express to extend it, as it couldn't reach to the far corners. I bought the new one, hoping it would be able to have a longer range and eliminate my need for the Airport Express. Good news: totally worked! There is noticeable range improvement. I have a Chumby on my night stand which is in the opposite corner of the home. Before, the Chumby couldn't connect without the help of the Airport Express. Now the Chumby can connect just fine! Same w/ my wireless photo frame, Squeezebox radio, and macbook! So the range was definitely improved for me.
Additionally, I didn't copy my backup from the old Time Capsule to the new. I just re-did backups on all the PCs before I blanked out the old Time Capsule. I notice that the new backup speeds are very much improved - not sure if that's due to a better drive or better internal hardware. And it's not just Time Machine - copying files to the drive on the TC is noticeably faster as well.
So that's my experience going from gen2 to gen3. I'd say the main perk for me is range. If you weren't having range problems, this isn't going to be a huge upgrade for you. But if you could use some extra range, this is a great help!
(oh and I was able to eBay my old Time Capsule + AEBS as a bundle for the same price as my new TC!)