I love this little thing I really do. I bought it for myself as a present for a professional milestone. I can't tell you that it is worth almost 200 bucks, that's your call, but I don't regret the purchase.
 
Pros:
 
Sound Quality- Versus my work PC headphone output it's night and day, no more noise, anemic bass, and almost mono soundstage. Versus MacBook Pro headphone Out definite improvement especially in bass and separation. Versus iPhone 6s headphone out a subtle improvement, slightly fuller and more defined bass, better guitar separation.
 
Portability- Some could argue that a high quality PC sound card would have improved my PC sound cheaper. Fair enough but I certainly couldn't have easily unplugged that sound card and used it on the MacBook Pro or the iPhone 6s. I have no problem folding over the Dragonfly and required Lightning to USB connector into my pocket.
 
Appearance- This thing looks the business nice shiny red with a not subtle glowing dragonfly.
 
Cons: 
 
Really none. Shy of a bit of heating up (less than the iPhone itself) this thing does not disappoint.
 
Comparison:
 
The only other dedicated DAC I have is my RME FireFace audio interface. Compared to that the Dragonfly it isn't really in the same league. The RME sounds like I'm sitting in the studio with the band. But I can't pack a full rack space device in my pocket. 
 
A little about my setup:
 
My primary listening is lossless files on my iPhone 6s >> Apple Lightning to USB 3 camera connector >> Audioquest Dragonfly Red >> Sennheiser HD380 Pro (with a swapped out cable)
 
The Apple Lightning to USB 3 is a must have for me. It allows for the charging of the phone while listening. It is slightly wider than the standard connector but that doesn't really matter to me because the phone is still twice as wide as the connector. Being able to charge is a must for travel in my book. 
 
A little about my musical tastes:
 
I tend to listen loud. I listen to mostly alternative rock and mostly hard guitar centric stuff but not metal, think 311, Smiths or Less Than Jake. I also listen to some more synth oriented stuff; Depeche Mode, NIN, Prodigy. Occasional hip hop mostly Tribe Called Quest.

I first used these as monitoring headphones for recording. They quickly moved out of my home studio and now I use them everywhere. I have one pair at home and one pair at work. I also take them when I travel. 
 
Pros:
 
Comfortable- I wear these all the time. They are a little tight which helps with isolation. Sometimes they require a bit of repositioning to get back to comfortable. 
 
Good isolation- Great for recording. Also great at work and for travel. It blocks out a lot of noise
 
Good sound- I like the sound of these. For me I find that what I hear through these head phones translates well to the car and speakers. 
 
Good durability- Two years of heavy not always careful use and travel not a scratch, chip, flake, or fade.
 
Replaceable cable- Nice to have flexibility.
 
Cons: 
 
Needs new cable- The flexibility is nice but it really needs a new cable. When sitting at the computer plugged into an audio recording interface the coiled cable is nice for movement. In all other applications it is just too heavy and awkward. Fortunately it is very easy to replace the cable. 
 
Bland design- Perhaps not a con for some but they are plain black head phones with Sennheiser conspiculously written in three places.
 
Comparison:
 
The only other serious closed back over the ear headphone I have used extensively are the Sony 7509's. I think the sound from the Sennheisers translates better to speakers and the car. I also think the Sonys are less comfortable. The Sonys started to fall apart after the first year. Mostly the pads started to disintegrate and the chord (also a coil) stretched, twisted, and kinked to an almost unusable level.
 
A little about my setup:
 
I replaced the cable with a Dolce and Gabbana 1.2 meter cable this cable fits perfectly. I also have a replacement cable from the B&W P5. It fits well and has a remote for an iPhone which is nice when I'm not using a DAC. 
 
My preferred listening setup is lossless from an iPhone 6s through a lightning to USB connector to an AudioQuest dragonfly red.
 
For music creation I use a Mac Mini with and RME FireFace as an audio interface. I am mostly a Propellerhead Reason user for DAW software. All of my Mics are either Shures or AudioTechnicas. My guitars are all Fenders.
 
A little about my musical tastes:
 
I tend to listen loud. I listen to mostly alternative rock and mostly hard guitar centric stuff but not metal, think 311, Smiths or Less Than Jake. I also listen to some more synth oriented stuff; Depeche Mode, NIN, Prodigy. Occasional hip hop mostly Tribe Called Quest.  The music I make is mostly goofy alternative, a harder They might be Giants maybe.