We use LSI MegaRaid 320-2x which is SCSI based in our FreeBSD 5.4 Server. LSI even has company support for FreeBSD. The card is the best.
I installed the Card and connected the SCSI backplane to it which connects the hotswap drives. I then entered the LSI utiliy to create an array. After this was done. I installed FreeBSD 5.4, there were no problems, nothing special to do, just installed it normally. It just appeard as one disk. I loaded all the ports, configured the server, Finally I tested.
I tested the RAID array be pulling out a hotswap drive while FreeBSD was running. I left it out, ran some commands, and changed some files.
Everything worked perfectly. Then I placed the drive back in and it immediately started to sync back with the other drive. I then rebooted the OS just to check. No problems. No performance degradation either.
I do recommend getting a optional battery for the RAID card. This will increase reliability during cache writes if the power fails. Of course this is not a specific problem to LSI all RAID cards that perform cache writes suffer from this.
Hope this helps. Here is the link to LSI