The down side of blended fuels is that your motor has to burn more blended fuel to perform the same amount of work as it would burning an unblended gasoline. The blending agents (ethanol, MTBE, Propane, etc.) typically have 1/2 or less the energy density of gasoline. Hence, your motor has to burn more fuel to accomplish the same amount of work. As Robert mentioned, there are some advantages to this kind of fuel (from an emissions standpoint) as far as easier starting in cold weather and earlier catalyst activation. The downside is you burn more blended fuel than you would regular gasoline to traverse the same distance. Leaning out can also be a problem if the carb you are using has fuel passages that are only barely keeping up with the idle and part throttle demands of your system on regular gas. This is not normally much of an issue at the levels these fuels are blended at (<30%) but can become a problem at higher concentrations of alcohol. For alot of us here in the states, we have no choice but to burn blended gas in the wintertime by law. If you can avoid it, I'd jump at the chace to save yourself a few bucks.
'66 candyapple red coupe, 347 4v engine, performer RPM heads and intake, 600 cfm holley carb, Sanderson short tube headers,
T-5, Versailles 9" w/ 3.50 gears and a detroit locker, 4 wheel discs.