I've used FreeBSD for as long as I can remember. My desktop is _still_ FreeBSD.
Having said that- nothing performs under high load situations like Solaris. I was not happy with Solaris in the 7, 8 or 9 days (Mostly because they kept changing things- seemingly at random), but Solaris 10, and especially OpenSolaris, have been rock solid and cool as can be under high load.
My current OpenSolaris heads are running on Dell 2950's with 32 GB of RAM, Myricom Dual Port 10Gb ethernet nics, 8 lane (2 x4) 3Gb SAS controller, dual port LP10000exDC Emulex 2Gb/s Fiber Channel cards, and dual 3GHz quad core processors. They're connected to 6 Promise vTrak J610s Dual controller disk shelves. The shelves have 4 32GB write optimized SSD's in a mirrored/striped config spread across both controller channels for our ZIL. There are 80 15k SAS drives and 8 256GB Read optimized SSD's as the L2ARC.
Both heads are connected to the shelves in an active/passive configuration in which the second head can immediately pick up if the first one fails.
We use COMSTAR to export ZFS volumes via iSCSI (over 10Gb ethernet to our VMWare servers) and Fiber Channel (To our older Sun QA Farm).