There are a couple of tradeoffs. Normally you think of a light twin as giving you relatively:
More reliability
More load
More speed
At a much higher fuel burn when compared to a single of similar size.
The Diamond doesn't offer much more in the way of speed or load compared to the DA40, so I think of it as a way to get the reliability of the twin (and ice protection) without driving up your operating costs much. Even though you have two engines, the engines themselves are disposable, and you get them (at least for now) on a "power by the hour" price strategy, where you are guaranteed a discount on the engine replacement if the engines don't make TBR of 2400 hours, or they don't succeed in getting the engines TBR to 2400. If you figure $50,000 for engines every 2400 hours, with less engine maintenance (they are simpler, mechanically), then the airplane looks like a good value if you are a travelling pilot as opposed to a more local-area type. What remains to be seen is:
Fleet safety record, and how well they do protecting people in "off-airport" landings.
Will the FADEC be maintenance intensive? Given how reliable automobile engine control systems are, I'm hopeful, but we'll have to see how it goes.
Will the prop/gearboxes be maintenance intensive?
If I had $500,000 to spend on a new airplane, it would be a tough choice between a turbo 182 (where except for the G1000 there should be few "surprises"), a Mooney Ovation (same issues) or the Diamond Twin. If the Diamond had the time in service of the Cessna or Mooney I'd buy it, even at the slower speed the extra engine is worth it to me for the type of flying I'd be doing. I'd take the Diamond over the Cirrii for the same reasons, they're both relative newcomers and two engines and 155 knots and known ice is worth more to me than one engine and 180+ knots.
