Dealer I spoke to yesterday said that most people with the 125i are getting around 33mpg when driving in the eco mode as opposed to the stated 42mpg. I checked 1 addicts and babybmw and this does seem to be what people are saying. Most of the cars are still quite young though so may improve.
Going on that I'd say that Megas guess of mid to late 20s for the 135i is probably about right. Not sure about what it might do on a length of the country type motorway cruse. Couldn't see it being anymore economical than the 125i otherwise what's the point of the 125i?