Yes, the XMEGA has some significant improvements over an ATmega.
I could be seduced by the fast 12-bit ADC and DAC.
On the the hand, ARM is comparable in cost, sometimes cheaper.
ARM versions with on-board USB end up being noticeably cheaper, though that hasn't shown up in board prices.
Some ARM have on-board ethernet, USB OTG, ...
Some ARM boot themselves over USB, and need no drivers installing on the host. They have driverless USB bootloaders manufactured in.
XMEGA two years ago would have been good.
But now NXP, ST Micro, Atmel, TI/Luminary, Cypress, ... have got some nice silicon. A friend uses PIC32 for everything, and loves it, and he is still a strong AVR fan, but PIC32 is better for his uses.
Don't get me wrong, I adore my Arduino's. I might get a good XMEGA Freeduino, if I found one I liked.
IMHO the technology is likely to fork if we don't support both lower cost and higher performance, and both of those can be done with ARM. I'm looking at ARM, and porting the libraries. I think the core work will be done before year end anyway, without my help.
But, It is nice to dream :