Haven't looked into FreeNAS - I just like the possibility of different size drives...and redundancy.
My requirements are quite simple, it'll be for media storage (my ripped bluray movies, dvds, music, photo's (I have well over 500gb of photos) - and I need to be able to survive a drive failure, I don't anticipate more than one failure at a time... and will take that risk.
The ability to use different size disks would be a bonus...
Performance is not a major issue, as long as its fast enough to playback a full bluray off the network.