Ethanol has less energy per volume than petrol. Using it will see a decrease in fuel economy. How much of a decrease will depend on a lot of things.
Ethanol has a lot of uses in fuel beyond "saving the planet" (something there is a great deal of argument over, particularly with corn based ethanol).
1- Ethanol is a very powerful solvent / cleaning agent. You can use high ethanol gas to keep fuel lines and injectors/carbs cleaner. I know of several motorcycle mechanics who clean carbs 'the easy way', but soaking them in a bath of pure ethanol over night.
2- Ethanol burns cleaner (no sulfur in it). Around here you can buy Sunoco (now PetroCanada) 94 octane gas which is a high Ethanol (E10) blend. This stuff both cleans very well and burns very cleanly. Run two tanks of it and pretty much any car will pass an Ontario Emissions test according to a friend who owns a car dealership.
3- Ethanol also lowers the freezing point of petrol. Which means that it is often added to "winter gas" to prevent gas lines from freezing. Even in pure 'petrol' blends, they have to add something to prevent freezing in winter climates, and whatever they add does reduce fuel economy (one of the multitude of reasons why fuel economy gets worse in the winter).
When not to use an Ethanol blend?
1- Some engines just don't run as well on the stuff. My 2004 Mazda6 V6 (3.0L Ford V6, same as used in the Taurus of that year) would get 2-3L/100km worse mileage on E10 gas than I would from pure petrol; going from 10L/100km (23mpg) to 12L/100km (19mpg) with no other change. It was very consistent with this (I tracked every tank). But, I have not had such extreme differences with any other car I've owned. In fact my '92 Mazda 323 ran better on E10.
2- Old fuel lines. As mentioned, ethanol is a great cleaner. To the point where it will eat through fuel lines and gaskets designed for petrol only. I don't think this is a problem for any car sold in Canada since at least the 80s (we've been using Ethanol in our winter gas for at least that long). This is more a problem for classic cars. The other solution is just to replace the fuel lines with something modern.
3- Plastic fuel tanks. I do not know of a single car that uses plastic fuel tanks, but they are not uncommon in motorcycles. Depending on the plastic, the ethanol can cause warping and eventual leaking of the tank. The only motorcycle I know of for sure that suffers this problem is the Ducati MultiStrada 1000. Ducati issued a recall where they would replace warped tanks with an original tank (which sucked for owners, because it just meant that the tank would eventually warp again). Since a Ducati isn't a very Mustachian vehicle, I doubt it'll be a big issue for most here (my Dad however owns one of the affected bikes, hence why I know about it).
4- E15. Many governments mandated that cars be able to handle E10 long ago. So fuel systems can handle it. E15 was introduced much more recently, and I believe without the consent of the auto industry. I recall reading a number of executives were upset because they expect it will damage their fuel systems which were only rated for E10. I would read your owners manual carefully. Mine specifically states not to use higher than E10. I would follow the advice of your owners manual.
So, in summary - yes Ethanol will most likely lower your gas mileage. How much will depend on a lot of things, it could be a lot (4mpg) or it could be virtually indistinguishable from other factors. It could even help if you are running an old high mileage engine. If you are running a classic car with old fuel lines, avoid it. Check with your owner's manual or contact your car's manufacturer (I'd go head office, not dealership) to find out about E15 before using it regularly.