Rich
I have loged about 50h in a C-421c but this was some years ago. Indeed, a great aircraft to fly. Other than fuel consumption the turbos are sensitive to shock cooling, also the gear reduction between the engine and the prop needs to be taken into consideration so you have to be very careful the way you operate the engines, also there are some issues with the exhaust system and I think that there might be some with the spar but am not sure if the 421 is effected.
If you can efford the running costs (i would say £500-600 per hour) and if you are looking to buy one than the C model is the one to go for.