Pros: open soundstage, airy presentation, sweet sounding
Cons: expensive :)
I have the Bifrost before this and plugged this into my Valhalla before my Mjolnir arrived. Even though it uses the same D/A chip as the Bifrost, I found it to be a significant upgrade that I could even hear with the Valhalla and my Beyerdynamic DT990 (600 ohm). If I could only choose between the Gungnir or Mjolnir for upgrading my headphone system, and my headphones didn't require the extra power, I would probably go with the Gungnir, it was a more noticeable upgrade. The Mjolnir was noticeable but unless your gear really requires that power, it probably will be less noticeable for you. Even with the single-ended connections I was using with the Valhalla, it was noticeable. I am using it at work. I connect my macbook to a M2Tech Hiface to the Gungnir coax port. If you get the USB version you shouldn't require the Hiface. I am using a Pure i20 for my old iPod to stream lossless to the optical port. This lets me use either the computer or the iPod with my Gungnir, and they both sound great. I have since upgraded from the Valhalla to the Mjolnir. My headphones have been upgraded to the Hifiman HE-4. I have recalbled all of my headphones to balanced and am not looking back.
Not sure if I feel hard-pressed to upgrade at this point either. The statement DAC supposedly includes a lot more power, but I think I could even upgrade to the HE-6 and still use what I have just fine. Bottom line, the Gungnir has really made me feel like I have taken it as far as I needed to take it. My headphone system sounds better than my home system now. So obviously the money needs to go towards upgrading the home system now lol...