You can do all sorts of really neat things. You can define pools of drives at different cache levels. You can have a bunch of mechanical drives for deep storage, some for hot storage, SSD to cache recently read files, then write-through from the SSD down to mechanical drives, either immediately or after a delay.

It's pretty much everything I could wish for from a filesystem, though I haven't actually taken the time to try it out yet. AFAIK it's still somewhat experimental, more or less in beta.