Well E-Series is block level only, and for what I have heard does that extremely well, and can very much outperform a FAS in SAN. Also they offer Infiniband, so they are more of an HPC oriented platform I think.
So FAS is a feature rich , multiprotocol storage platform that comes with the pricepoint of feature richness... If a point solution such as video survelance, backup target etc. is the oportunity for sales, and the budget is departmental driven or bond money (Public sector) allocated, then E Serise makes a great play since it will offer great scalability and performance at a lesser price point.
These are perfect points. There are "different types" of flash. There are systems that are meant for pure speed and there are systems that are meant to be a full features flash option. The FAS fits into the second catagory. The E-Series is going to be closer to the first.