I'm not sure on the exact efficiency differences, but I hear a lot of anecdotes locally about E10 resulting in 8-10% less range on a tankful than using straight petrol/gasoline (E10 is VERY COMMON around here, due to an over-strong farm political lobby).
The general rule of thumb that I've seen is that E100 has about 62% of the per-volume energy content compared with 100% petrol.
Ethanol is also VERY BAD for many small engines (lawn mowers, etc), due to seals that it will dissolve. Ethanol absorbs water and may cause faster corrosion of engine parts, too.
Mike