It's relatively uncommon. They made less of them than ATs for sure. It's basically an AT in an XT case.
It's kind of a cool system to own, because it was the last of the classic IBMs and it had a few pretty nifty authentic IBM expansion cards for it like the VGA display adapter, and a 12MB upgrade board that used 30 pin SIMMs. The motherboard is also somewhat interesting, because it was rumoured IBM gimped it down because they didn't want it to compete with the PS/2, but it was supposedly on reserve as a backup plan incase the PS/2 bombed.
I have a non functional XT 286 motherboard that I was planning to fix and use to pimp out my AT. Back in the day, I recall people having pretty good luck with overclocking these boards. I'm hoping for 10MHz (zero wait states). Some ATs in europe actually used the XT-286 board from the factory, and I believe it was internally referred to as a "Type IV" AT board.
"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium