I have no DA40 flying experience but looked at it seriously in 2002 (it was advertised with avgas back then).
Starting from brand new, and setting engine problems aside, I'd guess the ex-fuel operating costs would be comparable to something like a new TB20 i.e. pretty low for a number of years. The fuel bill would of course be much smaller.
The thing which I would query in particular would be warranty work arrangements for work away from the dealer. Obviously if AOG then one has no choice, but you don't want to fly it back to Gamston for everything.
Another thing is that if the plane has particularly troublesome components (the engine is one known thing but they may have genuinely sorted it out by the time you get yours - or they might have produced some new operating guidelines) these are likely to surface well before the warranty runs out, and in such a situation you have a very strong legal case for effectively extending the warranty for extra years, on those items. I have such a pseudo-warranty on certain avionics...
The above are the negative things I'd look for. The aircraft itself will fly just fine, I am sure
