I went from an RX8 to a 330d and to be honest I miss the fun of the RX8. Don't get me wrong I like the 330d (I'd have to I have done 20,000 miles since August in it and its saved me soo much money 20mpg against 40 mpg) but it doesn't have the smiles per miles of the RX8.
I am getting itchy feet again and feel a new car should come a long soon but money issues are are stopping me so I think a remap to 260BHP will have to suffice for the moment.
I don't want to put you off getting a 330d (or even a 530d) because they are fantastic cars but it just doesn't have the same drivability as the RX8. If you enjoy the feel of the power and the road presence of the RX8 you will be disappointed by the Beemer.
But if money is your motivator that go for the beemer and you won't regret.
Regard part-ex, mine was a 53 plate 230 with leather and I got 15,250 for it when it was 20 months old with 25,000 on the clock. I didn't think that was too bad as my sisters boyfriends works for a larger car chain in Essex and they only managed to get 15,000 for a similar spec (with sat nav) in an auction.
Cheers
Steve
Edit
Forgot to add if you can get an ex-BMW management car then you will get a much better deal from a dealer. I got a 54 plate 330d sport 6 months old with 6k on the clock for 10 grand less then the new price.