Not good, 23 - 25 for the London commute, upto 30 for other journeys.
I did not buy the car for it's mpg, or to drive it slow... So if I was a bit more gentle I could tease a bit more out, but why? GTIs just love being let loose. I use the wife's smart car 4 days a week for the commute as she takes the train. I'm probably going to turn about 5 - 6K a year so the the cost for fuel isn't great.
The 35 is great to drive and I love every time we go out. If your mileage is going to be low do it, if your worried about fuel cost try a mk7 I think they have a much better mpg stat.