I have now had my MCSD one month, and covered 1500 miles.
the average MPG as recorded by the Car Trip Computer is 51MPG
I have had 6 trips to full up with three top ups of less than 20 ltrs
2 of less than 25ltrs and one of 39 ltrs. so a total of 130ltrs giving a calculated MPG of 51.5MPG
This has been achived with a mix of driving with a low of 47.4MPG covering 240 miles which included a bit of a motorway thrash(>80mph)for 70miles.
A high of 56.1MPG over 185 miles mainly sensable motorway driving (circa 70mph).
Here's the fill breakdown
21ltrs 237miles 51.3MPG
23ltrs 240miles 47.4MPG
16ltrs 178miles 50.6MPG
16ltrs 192miles 54.6MPG
15ltrs 185miles 56.1MPG
39ltrs 440miles 51.3MPG
:D

I've had mine for 3 weeks, have done 1600 miles. Each time I fill up the MPG gets better - Last tank gave me 55 MPG and I have a heavy right foot!