I've used it in the past (I have a client in SF that use it) but I have not used Postgres on it. The theory as I understand it is the kernel is faster and the I/O is also faster than Linux. But I don't know this to be true. The client in SF uses foxpro for Unix and discovered a major increase in performance. However, he went from 400MHz SCO Unix to 1G BSD. So who knows what the cause of the increase in performance.
John