The DA42 burns almost the same fuel at almost the same cruise speed as my TB20 i.e. 11 USG/hr at 140kt IAS, though of course for the 42 this is the total fuel burn for both engines.
The reliability scenario seems to be improving but whether a private owner can consider the engine technology to be "proven" after the debacles of years past, is a personal view. I'd like to see a few more years. All DA42 private owners I know have been extremely critical of Diamond's handling of the issues and would not trust the company as far as they could throw it.
One Diamond maintenance shop I know tells me to not even consider buying a DA42, due to the amount of stuff they are forced to replace during each Annual, due to simple corrosion.
Commercial owners are mostly schools (FTOs) and they are notorious for down-playing any downtime because it makes them look bad to do otherwise.
Another factor for the UK is that on flights other than commercial or "training" (no idea if "training" has ever been defined) you are supposed to self declare the avtur duty, and that really kills the fuel burn cost advantage of diesel.
People who rent them generally like them, and having flown in a DA42 I can see why. It flies very nicely.