Just got my Explorer a few hours ago (from Audio Salon in Santa Monica; highly recommended). I've had a Dragonfly for a week or so. One thing I confirmed immediately is that you can feed headphones and an amp/powered speakers simultaneously via the two output jacks. That's an awfully nice feature for a desktop rig with both headphones and speakers, for example. You don't have to swap between one 3.5 mm connection (as with the Dragonfly), not to mention RCA jacks (as with a Modi). You can even listen to both at the same time, not that you'd want to, but it makes it really easy to toggle listening between 'phones and speakers. The iDAC, for instance, has both headphone and RCA output ports, but you can't run them simultaneously, at least not correctly, I think. Of course, to actually implement this on the Explorer, you'll have to rely on the Explorer's built-in amp and do without a stand-alone headamp, since the headamp would require the use of the same analog out as the powered speakers. Otherwise you're back to swapping cables connecting with the non-headphone out. Even if you assume that DACs are hard to pick apart on sound, this functionality seems to distinguish the Explorer.
The other early observation is that the Explorer holds up better than the Dragonfly at high volume. By my calculations and listening, the Dragonfly delivers a bit more output power into my HD-650s. But the Dragonfly sound falls apart on most recordings below max volume, and can be fatiguing even modestly above 50%, even though the loudness level is otherwise reasonable between 50% and 75%. The Dragonfly sound gets unpleasantly harsh just before I reach the threshold of discomfort induced by volume alone. The Explorer doesn't go quite as loud, but seems to remain listenable louder than the Dragonfly, closer to my maximum loudness threshold, and I can reach that threshold on many recordings. On other recordings, I could see the merit in an external headamp to get up to eleven, so to speak. Running each DAC through an amp may level this playing field, of course, but part of the value proposition here is that you're getting a headamp in the package, and in the case of the Explorer, one you can use alongside the line out. Accordingly, listening at milder levels also narrows the performance gap, but the Explorer has yet to induce any fatigue.
Any comparative evaluation beyond that at this early stage (and probably at later stages) I'd attribute almost entirely to expectation bias. What I hear so far does confirm those expectations, that the Dragonfly is lighter and leaner than the Explorer, that the Explorer delivers more of the musical whole than the sum of the parts, whereas the Dragonfly picks them apart. I even detect better imaging through the Explorer, which is not something I recall reading about beforehand. I'd probably like something fat and rich like the iDAC even better, but don't feel compelled to go there while listening to the Explorer. And I tend to believe that you want the DAC and amp to be neutral and transparent, whilst seeking sonic character via speakers (or even EQ, if necessary).
Thanks to shuffle, I've cycled through choral, opera, and string ensemble while writing this, and it's just gorgeous. Not that the Public Enemy and indie/classic rock crush tests beforehand weren't. It's very early, but I don't see the Dragonfly winning this battle for my listening.