I am sure aero diesels will be considered "proven" one day but it is too early for me
Well, not so sure about that from the technical reliability point of view. Diamond do have a
good safety record and a lot has happened since the first Thielert engines went into service. I haven't heard much at all about issues with the Austro Engine. I have heard a lot about bad cylinders, crankshafts, camshafts, valves, etc. from Lycoming and Continental. I have at one point considered a Cessna P210 (pressurized) but after having found out how easy it is to cook the engine, I realized that one can not trust the previous owner of such an airplane.
The main reason I don't have a diesel aeroplane is because there isn't any that would fit my needs. The Thielert and AE300 currently address the entry level market with 135-165 bhp (180 in the DA52 prototype). If I was considering buying a new C172, I would certainly get something with a diesel instead.
Airplanes like the 182RG or TB20 which give you long range, high altitude and good speeds are no longer in production and there is currently no diesel engine that can deliver 250bhp. Once Diamond get to certify their 6 cylinder diesel and start shipping the DA50/DA52, things might change. It would be great if Cessna restarted the C210 but word is that they wouldn't even consider getting back into the retractable gear business.