Pros: Price, Reputation of the D1, Bit-perfect USB support out of the box
Cons: Ground Loop
I got the D1+ with all the upgrades after reading all the awesome reviews about the original D1 on head-fi. It was pretty clear to me that the headphones I received for Christmas (Sennheiser HD 650) weren't going to work well with a 3.5mm jack out of my laptop. So after hours of researching the best options for my budget (250-350 bucks USD) I went with the D1+ and hoped for the best.
After unpacking it and hooking it up I played a few songs with Foobar2000 and was very pleased. I've listened to hours of music now and am still pleased with the unit, hearing micro-details in Pink Floyd and The Beatles as well as diving into albums I would other-wise not care about (good kid, m.A.A.d city, Koi No Yokan, The Lumineers).
You can really tell how well the album was engineered and even mastered from this combo, it doesn't forgive poorly recorded materials that well (it doesn't destroy the ability to listen to them though).
My friend who is in some form of music major and a huge audiophile came over with his Grado sr325i cans and plugged them in. He said the D1+ sounded better than the amps they used in his classrooms and better than his current home setup.
The investment really seems worth it, I can also buy a better amp if I ever feel like it (might not ever happen) and use the D1+ as a standalone dac/preamp (I've read the tube preamp is subtle and still stays true to the source material).
Main problem is I don't really have anything to compare it to in person. I won't be upgrading until I actually hear better, not just off reviews.
So I recently built a new desktop computer and moved my audio setup over to it. I noticed a hissing sound in my headphones and thought it was a problem with my computer at first. Finally, after hours of trouble shooting I figured out it was my amp. I was using the Maverick Audio D1+ through USB back when I had a laptop and was still using USB for the desktop. Apparently, this created a ground loop and was easily fixed by using an optical cable. Just something to think about if must use USB.