I knew before I bought it what I was in for fuel wise. 
For day to day use I never go near a Motorway or anything resembling an A  road, so it is all town driving, allbeit traffic free. Normal driving  with the odd blip here and there and I get around 22mpg. If I'm extra  carefull I can see 25mpg. When hooning I'm happy if I see 13/14mpg.
Done a couple of decent motorway trips, 40 or so miles each way. Cruise  set at 70 it will do 30mpg all day long. On one stint I set it at 60 and  it indicated 35mpg. 
Those are all trip indicated, cba to work it out myself.
If you are worried about fuel I would stay away from one, but if it  isn't too much of a concern then I can hand on heart say that 20mpg is  worth it for the noise alone!