I've gone through FreeNAS 0.7, OpenFiler 2.3/2.99, and somehow i ended up on Scientific Linux 6.x. FreeNAS seems to be concentrated on the interface more than the stability of the system and safety of your data. FreeNAS 0.8 was even less stable than 0.7 somehow (WTF? shouldn't software iterations make things better?).
Openfiler 2.3 didn't have the hardware support I needed. 2.99 had the hardware support, but the stability was horrid.
So I went with a vanilla Scientific Linux 6.0 (newest at the time), and I havent had a glitch since. RAID1 for startup disks, iSCSI exporting LVM2/software RAID-5 over 5 disks. It's so stable it's boring.
Since then we have FreeBSD 9.0 that just came out few weeks ago, so that might be a good option, as long as it supports your hardware. I've used older FreeBSD's for servers in the past and they were very nice, so if 9.0 is of the same quality, it might be a nice option.
There is no one clear answer on these sort of questions, you usually end up picking whatever is stable enough that runs on your hardware, and fits your use case (ZFS over HBA's, or hardware RAID with LVM on it, do you need iSCSI, etc...) The best advice I can give you is boot them all up, and just see what boots first. That usually narrows the field quite a bit, making the selection process much shorter.