My only suggestion would be to make sure you test recovering from lost drives with FreeNAS before you put your important data on it. And (as always) make sure you have good backups. FreeNAS' forums have quite a few threads about lost data with no good resolutions other than "well, that'll teach you to keep backups".
Just make sure you can actually do things like replace a dead RAID disk before relying on it. It will save your bacon later.