I run the MGB on E10 all year round - don't have any bad effects, and average about 28-30mpg. With the Morgan and Harley I tend to stick a tank or so of E5 in at the start of the season, then run on E10 but try and overwinter with E5.
Contrary to a certain opinion several posts above, Fuel mileage is important - if I ran the MG on superunleaded it would be an extra £20 or so a month. I rather spend that money down the pub. It's also one of the reasons I went for a 4/4 originally - I wanted to be able to drive it all year, any distance without worrying too much about the cost of fuel. (I get about 38mpg).
My driving is pretty unsophisticated, so wouldn't really notice any performance difference between E5 and E10
