I had a Dragonfly for 3 weeks. I agree with Peter_s; it matches up nicely with the Sennheiser 580s. It's really nice to get this really clean sound, some oommph behind the headphone signal, and analog volume control that doesn't toss away bits when you turn down the computer volume.
I took it back because--although I really liked what it did with headphone listening--I was looking for something to take the edge off the sound when I play lossless iTunes files through the stereo. This is nothing against the Dragonfly, but the things that made it work for me was installation of Audirvana Plus software, setting the upsampling to double the original sampling rate (so, for example 44.1 Khz files were upconverted to 88.2 Khz instead of 96), setting it to "hog mode" (i.e., turning off warning bells and automated backups), and using a sizable part of RAM to buffer the files before playing them. I keep my iTunes files on a portable USB drive, and until I used the Audirvana buffering, the sound was somewhat bleached and definitely not very dynamic. The buffering fixed that and then some.
For the $50 of Audirvana Plus I got what I was looking for--up to a point. The icing was when I swapped out an opamp line stage for a full tube line stage. That took the last of the digital rough edges off what I was hearing at the speakers.
I had already returned the Dragonfly before I changed line stages. I think that with the better line stage the Dragonfly would have added sparkle, clarity, and dynamics without the harshness inserted by the opamp-driven line stage.
The Audirvana is the inexpensive way to go for Mac owners. In addition to everything else I mentioned, you can use it independently as a music player or as a plug-in for iTunes. It also enables you to play FLAC files, including the hi-def downloads from HDTracks.
For Windows users, according to TAS J.River Media Center (JRMC) is the way to go and it largely provides the same sorts of controls and improvements for Windows as Audirvana does for the Mac.