B61, Yes I am referring to the DA42s. They are brand new so have not yet reached an age whereby engine and gearbox problems raise their ugly heads. Moreover, they have the new engines so are unlikely to suffer the same problems as the Thielert ones did. However, the savings in fuel costs are, as you said, far outweighed by the cost of getting them refueled (30 Euros per aircraft each time plus the cost of the fuel) plus the need to refuel them after nearly every flight.
The DA 42 is a nice aircraft to fly but has far too many limitations and, in some ways is a victim of its own fuel efficiency. Were you to get airborne at FTE at max AUW you would have to fly for nearly 2.5 hours to get down to max landing weight. This means limiting the fuel uplift hence the need to top up after most flights. The C of G is a long way forward which has its own limitations, especially in icing conditions. With the deicing fluid tank full the C of G moves a long way forward (well in excess of the max permitted in the vast majority of cases). However, when flying in known or forecast icing conditions, the maximum permitted forward C of G moves a fair bit to the rear!!! De-icing fluid is not cheap and could well push the operating cost up in winter in addition to the cost of fluid used in the summer months just to keep the system primed.
The Senecas are expensive to run but they do at least offer the flexibility of being able to get airborne with full fuel and three on board. With the DA42 limitations it is almost impossible to have more than two people on board.