I have one now as well. I'm coming from a NuForce Icon-2. The D3 is much brighter, assuming brighter means that the highs are much more pronounced and detailed. My mind was sort of blown on how much of a difference there was between the two. It is almost too much (the highs), at first it seemed like it just lacked bass compared to the NuForce but after more listening I think it is just the NuForce that lacks the highs. A track containing acoustic guitar made this very obvious when switching between the two.
The headphone amp is definitely much more capable than that of the Icon-2 and it has no problem driving anything I've thrown at it (HD580s, K701s, D990 Pros (250 Ohm), AHD2000, and SR225s) . That was actually the main reason I wanted to get a new amp, the Icon-2 has trouble powering the AKGs, Senns and Beyerdynamics, the Topping does not. Also, at low volumes the balance seems to hold, unlike the NuForce. Though, honestly, that was very rarely an issue for me with the NuForce, still, if you like to listen at low volumes, then that won't be an issue here.
The build quality is quite good too.
So far I am happy with it.