I only have extended experience with the Nord NC500 Mk II monoblocks. I have found these to take a LONG time to burn in. They really started to sound there best after several months of use - maybe 1000 hours or so. Anyone that comes to the conclusion that class D sucks after a few days or even a few weeks of use (assuming a new unit) is not getting a true perspective of what they are capable of.
I find these amps to provide excellent, maybe even exceptional, value. They are very quiet, detailed, dynamic, have great bass, lots of headroom, and, of course, are lightweight, cool, and efficient.
But I don't think they are for everyone. They are very neutral sounding - some would call this sterile. They don't have the gutsy upper bass and sweet midrange that many class A & A/B amps have. But a lot of this has to do with system synergy.
I recently acquired a Parasound JC-5. This amp is not lightweight, cool running, or efficient. It's also more than double the price of the Nords. But it does have a more pleasing sonic signature in my system, and a wider and deeper sound stage. To me, this is worth the cost, size, and lack of efficiency, but I can certainly see the Nords being a great fit for many audiophiles. Once fully burned in, they are really pretty damn good.