Genuinely curious. How does FreeNAS work with OS X? Have you tried it yet?
Amazingly well. FreeNAS supports CIFS/SMB and AFP among others so you can share files with pretty much any platform you can imagine.
Also, when you're setting up an AFP share, there's a check box that just says "Time Machine." If you check the box, when you can use OSX's built in Time Machine backup service to back up to that share. You don't have to dedicate an entire drive as I'm told you have to do with the official Apple hardware and you can even set up quotas for the dataset that the backup is on so that Time Machine will autoprune it and it won't get out of control.
I used Windows Home Server for a few years, then tried Amahi, then rolled my own server, but I'll never go back to any of them since I started using FreeNAS. I don't think I've run into a single sane thing I've wanted to do that I haven't be able to do yet. Be warned, though, that ZFS is one heck of a memory hog. That being said, I'm running my system with 6.5 TB of storage on only 4 GB but I wouldn't recommend it. I'm going to upgrade the system as soon as I have the free cash.