Well I don't have a big baseline to compare as I've only done about 2000 miles in mine since I bought it about 3 weeks ago, but it works out as...
Constant 70 on the motorway - just shy of 51 MPG
Constant 80 on the motorway - just above 46 MPG
A couple of mixed trips of around 350 miles in total, of which around 300 miles was 80+ motorway and the rest a mixture of spirited A road driving and being stuck in near stationary traffic for several miles - 42 to 44 MPG.
It has never run rough or sounded at all unhappy in slow or stationary traffic and I've never driven a diesel that has done so either. Definitely something wrong with one that does!
These are the MPG figures the info centre has shown - comparing with actual measurements of fuel put in and mileage travelled, it's never been more than about 0.6 MPG out. Over the whole amount of mileage I've done since buying it, my average is 46.1 MPG which I find pretty impressive for such a sprightly beast. Certainly beats my old diesel Volvo V40... and is quieter, faster, more powerful, holds the road better, looks sexier and is just generally a damn sight more fun to drive. Plus the growl when you put your foot down gives me the horn.