I'm right now not using it at the moment but I've researched the bullocks out of it to give a relatively informed opinion.
Basically, it's real nice.
You can use different hard drive interfaces to combine into one storage pool, which you're able to do a parity or two or three way mirroring. From what I've been told, the parity is a good option to for ULTIMATE redundancy and protect against drive death. But, it appears it's slow as crap to do that, around I think 20-30 MBs/sec. You'd use that for data that doesn't change often, like videos. Two way mirroring is faster, usually around the hard drive's native speeds, and copies your data twice across the pool. This would be used for maybe documents or pictures, depends on how far you want to go.
There is also thin provisioning where you can virtually set a pool to five TBs, but only have 1 TB physically available. As you reach the physical capacity, Action Center will tell you to add more drives to the pool.
The reason I'll be doing it is because I'm a media whore basically. I'm planning on an initial six TB setup with a hard drive cage meant for a RAID as it as alarms to indicate hard drive fault, which Storage Spaces works with as well. If the system detects an impeding death, it will ring off an alarm on the cage. I also demand drive fault resistance as I had a drive die on me and I didn't set up a backup soon enough. Couldn't recover ANYTHING. With that setup, I believe I'll start out with a two way mirror, and then grow it out to a parity for the videos. The benefit here is that you can grow it easily, where as if you did a RAID array, it'd be a tad more technical and tad more complex to do so, but that would be only for an internal drive setup, not external hard drives.
It's beneficial as it's based off Drive Extender from Windows Home Server 2008 I think it was. And it's also superior than it, and works rather decently as I've yet to see problems crop up anywhere on the interwebs. There's a thread I made a while ago about Storage Spaces and a response on there was REAL in depth, I'll see if I can pull it up as that can REALLY help you out.
I've played with it, but I'm not really to a point where I trust it. There is a lot that just doesn't make much sense with respect to way it works, and how it uses/misuses, or inefficiently uses hard drives.