We have been using Postgresql for many years now...
We have always used it with the native OS it was build from, FreeBSD.
FreeBSD is rock solid stable. Very reliable.
With so many rumors about Linux being faster especialy the 2.6.x kernel, I have decided to give it another try. I have not used Linux in 6 years. The last linux I used was 5.2.
I tried slackware 10.1 and Gentoo 2005.0 both at the same time.
I have used different file system seems rumors has it that JFS is faster than ext3.
After running benchmarks after benchmarks, I concluded Linux 2.6.x kernel is indeed faster. Without much details I would guess Linux is about 30% - 50% faster according to pgbench.
Furthermore I like Gentoo very much since it allows lots of optimizations while compiling.
Stability was not as good as Freebsd. Under heavy loads I have seen hangs and riad drivers are not as stable as Freebsd.
Server performance between intel and amd was also very different. Intel server performance for both pentium 4 and xeon was about the same between Linux and BSD. But AMD cpu shows large speed improvement under Linux.
This is just from my experience that I would like to share for other Postgresql users.