FreeNAS is based on m0n0wall which I use a lot so it would normally be my first choice.
However, I chose Openfiler for the AD integration and the fact that I can SSH into the box and play on a Linux kernel. I am not comfortable with FreeBSD which FreeNAS runs on.
So far Openfiler has been great. I have both NFS and SMB/CIFS shares with the SMB shares using AD integration. It also has a local LDAP server if you want to maintain users locally. Clients using the NAS include Vista, XP and VMware ESX. A minor niggle is that controlled/guest setting is per share which means that you cannot have a guest NFS share and a controlled SMB share on the same path.
You can do most things with the GUI but for some you'll have to SSH into the box, e.g. I wanted to migrate the software RAID level from 1 to 10. All in all I was impressed by the GUI which even let me setup NIC bonding.
We had a problem with backing up since we have a lot of multi-gigabyte files which was impractical to transfer over our company network which still has some fast ethernet components. To backup 1.5 TB, we plan to plug in a 2 TB USB drive to the Openfiler server, mount the drive and use rsync to copy changed files.
I simulated hard drive failures by swapping drives. Recovering from a boot drive failure meant reinstalling Openfiler and then reloading the configuration. It is very important to regularly back up the Openfiler configuration file for this reason. Recovering from a failed hard drive that is part of a RAID array requires using a SSH console to add the drive. The array will automatically sync the new drive. During this process the array is fully available.