If you want to know how well it works how about asking the one(only?) company that is shipping (what appears to be) block based automatic storage tiering which is Compellent ?

On the file-side of automatic storage tiering there is BlueArc, my company used to be a BlueArc customer though we never got to trying that particular technology from them.

My company's workloads don't demand automatic storage tiering, our work loads are pretty static, though we can change tiers on the fly, it's not something I've needed to do(though I certainly wouldn't turn down having the option). Our array is 100% SATA and gets slammed pretty good though the cache on the controllers is holding up quite well.

I have no experience with Compellent storage systems though their tiering technology has always interested me since I first heard about it.

Old news - Sun has had it for quite some time. Get the blueprint from http://wikis.sun.com/display/BluePrints/Deploying+Hybrid+Storage+Pools+With+Flash+Technology+and+the+Solaris+ZFS+File+System

The cache warm-up will take longer than 20 minutes, though.