I'm still preparing my review of the Sun Ultra 20M2 (in fact, I'm writing it in the other tab as I type this), but there's a really good summary of the performance of the beast: Geek Patrol | Sun Ultra 20 M2 Performance.

The figures compare Solaris and Windows on the same box and show some interesting differences in the performance of the two OS on what is otherwise and identical box. For example, these results show that bzip2 compression is much faster on Solaris compared to Windows, yet decompression with bzip2 is faster on Windows than Solaris.