It's terrible on fuel, their might be some circumstances were you get ok mpg but it's all relative. I remember when the cars first came out and was somewhat disappointed at the time, I loved the 70s Montreal and the Brera looked like it could be a modern day version. I liked the looks but the cars were very heavy which affected their performance and Alfa marketed them as sports cars when they were far more suited to being GT's
That said, time will change the way the Brera is looked at and things which are important on new cars become much less so when cars get older and move into the classic world.
The mpg on these cars are poor sorry but their is no escaping that, mid teens around town and if you look at the kind of performance on offer from a lot of cars with these kind of fuel consumption levels you can find some pretty serious performance.
However no one buys classics for fuel economy and running costs or even outright speed, which is why I think the Brera will do much better in the years to come as a classic car than what it did when it was new