A few years ago we were faced with the same decision, except with Facilis in the mix as well. We were a Unity shop and that system was literally bullet-proof. I had experience with Facilis from another job and thought the same of the hardware, but not of their client software. The sense I got from EditShare was that the software layer was overcomplicated for our needs. In education, if there's any way to break something (intentional or not) the students will find a way. Knowing that, seeing all 3 in action, we went with ISIS 5000.
We're likely adding another chassis later this summer. Its bullet-proof just like the Unity. I've had students beating on it every which way and its never crashed or hiccuped in 3 years. We run Avid and PP on it, in the same workspaces, and its great. Quite honestly if I was a Premiere-only shop I'd use ISIS storage. Its well worth the expense to me.
The facility I work at has been using EditShare for nearly 5 years. We bought it in 2009 when EditShare was really the only system that played nice with both Media Composer and FCP.
It can emulate a Unity. Their tech support is great. If you have to swap in your spare hard drive, they will overnight you a replacement drive.
The other question you need to ask yourself is "Do I want any of this goodness that Avid is promising as part of Avid Everywhere?" Some of that goodness, such as metadata indexing and searching, will only be available to customers who have an Interplay and an Isis.
Jonathan S. Abrams, CEA, CEV, CBNT
Apple Certified - Technical Coordinator (v10.5) Support Professional (v10.6 through v10.8)
Vice-Chairman, NY section, AES