I was a huge fan back in the 4.x - 6.x days.

The problems, for me, were: - apps developed specifically for Linux. The shim wasn't 100%. - Hardware support. This wasn't too bad, but I always had to take it into consideration. - Virtualization. This was the killer for me. Yes, jails are fantastic, if your entire workload ran on FBSD. But if you needed even one Windows/Linux instance, for any reason, you needed another physical machine. With Windows and Linux I could consolidate everything into a single machine.

Things have improved a lot, and I may try it out again, but unfortunately the mindshare is all Linux now.