I'm very curious to hear about the first impressions in flight...
I have about 400 hours on the original DA42, and about 60 on the NG.
The AE300 engines are about 35kg heavier than the Thielerts, mostly due to having a cast-iron block instead of aluminium. This allows for a higher compression ratio, and a bigger turbo, which yields better specific fuel consumption. The troublesome clutch damping of the propeller has been replaced with a visco-damper, and the idle speed is about 100rpm lower.
The additional weight has meant that the undercarriage has had to be modified, and the MTOW and MLW have both been increased, so although the aircraft is heavier, the useful load has been increased by about 40kg (I can't remember the exact figures now without looking them up - but they are approximately correct).
The COG of the aircraft has also moved forward by around 5cm, slightly ahead of the point which was intended by the designers.
You notice the difference from start-up. The motors are much smoother and quieter at idle - it is perfectly safe and comfortable to start the motors with the canopy fully open, and you can carry on a conversation with the pax.
Directional stability in taxying is slightly diminished, due to more weight on the nosewheel.
Run up is slightly simplified in that the FADEC has "voters" rather than manual/auto selection, and the FADEC and prop auto-test procedures are smoother.
The extra 20% power is clearly noticable in the take-off run and initial climb. But whereas the Thielerts had no limitation on using 100% power, the AE300s are limited to 90% after approximately 5 mins (again, I'd have to look it up to be sure of that figure). The oil is cooled by a heat exchanger with the main engine cooling, rather than with its own radiator, so oil temp climbs rapidly in the climb.
Whereas the Thielert's were unable to generate full power from an altitude density of around 8000', the AE-300s maintain full power up to about 13000' due to the bigger turbo.
Stall speeds, flap limiting speeds are all slightly different by a couple of knots.
Single-engine performance is better, but the asymmetric rudder forces required are noticably higher, despite the forward COG. However, N-1 is still far more benign than any other MEP aircraft I know. A big disadvantage for FTOs is that in simulated N-1 with one engine in idle and the prop in a transparency position, the gear-warning horn continues to sound (previously it was silenced) so all single-engine training will have to conducted with the gear horn blaring - not ideal.
Specific fuel consumption is better by about 10% overall, so you can either cruise faster at the same fuel consumption of the old aircraft, or use less fuel at the same speed. I'd have to dig out my notes to give the exact figures.
The NG also comes with some nice upgrades to the G1000 - Jepp cards on the MFD, Safe-Taxi (in the USA), synthetic vision, a decent VNAV setup, offset airways and the excellent GFC700 autopilot.
There are lots of minor improvements to the build quality and ergonomics - for example the fuel cut-off protectors are still spring-loaded, but click into place, so it is possible to cut off the fuel with just one hand instead of two!
The AE300s were tested ruthlessly at LOAN - they were forced to run on a testbed for hours - and they do appear so far to be a big improvement in reliability.
I have to admit that I mildly prefer the handling of the Thielert aircraft, but the NG is overall a much improved aircraft.
I hope that helps.