If you are refering to the latest incarnation of the mega 0 series (original X-mega) I think they have done well there. Now at last an AVR that OK can do 24MHz, maybe no one asked for that but they will work down to 3V at full speed unlike the older AVR stuff and even the mega-0 they introduced which while based of a 3.3V osly xmega had the traditional speed limitations at anything but 5V. I now see why they were having A and B type peripherals as now C and D types have emerged. I've not used any but they look like a worthy replacement to the mega 0 serieos and they won't require code rip up's for the new stuff.
Were x-mega SPI?
SPI, as in PDI or ISP? Yes. My ancient AVRISPMKII handles XMEGA.
UPDI is just async serial, bidirectional on a single pin. It doesn't require a programmer, just a UART, and a resistor to resolve direction. The comment above yours, I think is either assuming too much about the new protocol, or trolling. (Personally, I've tested it with a new AVR-DA chip, and was pleasantly surprised to use a MAX232 of all things, as a "programmer".)
To clarify, XMEGA is 1.6-3.6V, with frequency derated down to 12MHz at 1.6-1.8V, up to 32MHz at 2.7V. The onboard PLL can multiply up to 128MHz (derated to 48MHz at LV, over the same voltage range), and is used at that rate for timers and some other peripherals, while the CPU is limited of course (I haven't tried overclocking it).
AVR-DA is 24MHz (48MHz PLL for TCD only) over the full 1.8-5.5V range, at least, as near as I can tell. Interesting that it isn't dependent, or at least I've just not been able to spot the curve; it's a big datasheet.
I think there may be a number of false claims out there about microchip as haters like to hate. One user here told me that the XC8 compiler is just a rip off of the AVR-libC and GCC, well no not really, it was atmel that originally forked the compiler and libraries and when you install atmel (now microchip) studio you were just installing a commercially provided for free version. Sure they are probably not all nice but I don't think they are as bad as people make out.
The programmer on the new devices is the very same ICE introduced by atmel, they just put the price up although at the moment if you can buy one at all just shut up and be happy you can buy something
It's also integrating with PICkit and various onboard (demo board) things. The tool is open and in active development:
https://github.com/microchip-pic-avr-tools/pymcuprogIt was a bit of hair-pulling figuring out which tool to use (conclusion: pyupdi shows up first in searches, but it was more of a demo, with pymcuprog being the integrated and well maintained package; well, the documentation wasn't the best, but it's improving).
Tim