Headphone-wise I have tried most of mine (see list) and think they sound fine. If I'm at my computer "station" with the M-DAC, and no dedicated amp nearby, I have no problem using the built-in output, I don't feel like it would deprive me in any meaningful way of enjoying my music.
Yes, in absolute terms, an (expensive) dedicated amp may yield better results (esp. via balanced) depending on the headphone, and how finely tuned that amp is to your preferred sound signature, but poo-pooing the M-DAC amp section just because it is integrated would be doing it a disservice.
Some more specifics: the Audezes would be pushing it (but no notable distortion at max. volume, to its credit), the Fostex TH900 is my personal favourite (on the M-DAC, not overall), the W3000ANV is OK but prefer other options, it keeps the Edition 8's treble under control well enough compared to some other options I tried, the Senn Momentum still has too much (even a bit more?) bass, but even Tzar 350 IEMs work very nicely, with almost enough bass (on portable solutions, I tend to EQ in a bit of a bass boost, its only weakness IMHO)
As I mentioned elsewhere before, if I had to move and only keep the essentials at hand for a while, I would be perfectly content with just the M-DAC and my favourite pair of cans.