I've had my 325d since June last year and have just completed 17.5K miles since ownership. The car was 6 months old (dealer demo) and had 3.5K miles when I bought it. The car has just had it's first service at 21K.
Over the 17.5K I have consistently achieved around 45mpg. When I'm not so heavy footed I can easily reach 50mpg on motorways (sometimes between 51-53mpg) when using Cruise control set to 70mph. My commute is 110 round-trip each day - mostly motorway (M11/M25)
I previously owned a Golf 1.9GTi TDI PD 150bhp which loved to be 'ragged everywhere' and averaged about 45mpg. Since owning the 325d I've adjusted my driving style as the 6-cylinder engine is more refined and delivers the power more consistently than the Golf ever did.....Overall I think the 325d was a good compromise over a 330d vs 320d.
Although if I'd followed my heart I would have bought a 335d, but would choke on the fuel costs......