I get an average of 21.1mpg (OBC reads and average of 21.4) - that's a good mix of town and motorway driving. My best on a single tank is 25.4 (309 miles), worst is 12.8 (150 miles).
I found the 18's a much smoother ride (both on PS2's and Vred winters), but they just don't look as nice.
My running costs are approx £9k per year (including £5.5k for fuel and an extended warranty) - this does not include any depreciation.
E92 M3, Jerez Black, Fox Red Novillo, 19'' M Double Spoke, folding mirrors, lumbar support, front seat heating, PDC, USB, DAB, EDC, Bluetooth, Logic 7, High Gloss Shadowline, big smile