Ethanol takes ~3% off your FE for every 10% concentration. So 50mpg on MTBE fuel becomes 48.5ish. Talk to the guys in Chicago/Milwaukee who have been dealing with E10 for years...

http://www.epa.gov/otaq/regs/fuels/ostp-3.pdf

I personally verified the effect at 10, 20, 30, and 40% ETOH in my car. 3, 6, 9, 12% reduction in FE, respectively.

Yes, Colonial Pipeline (the primary source of fuel up the eastern seaboard) stopped shipping MTBE-laced fuel a month ago. So if your locality still requires oxygenation, E10 is the only option (MA is an opt-in area for RFG, so its locally-driven, not Federally mandated). Or... the fuel companies are voluntarily adding ETOH to stretch the gasoline supply.

I haven't determined what Maryland is going to do yet- but I imagine I'll get straight gasoline since we're a Federal non-attainment area.