These come so close to being my ideal headphones. These headphones have nearly everything I want, good controlled bass that extends well to very low frequencies. The bass is also present but does not dominate. Nice mids that are not recessed, a great wide soundstage and fantastic separation. The comfort is supreme, the earpads surround the ears well and the T1 is also lightweight. However, where it fails IMO is in the treble. Without applying EQ it's just too bright for my tastes.
 
I listen to music using lossless files and watch movies/TV shows using Bluray so the quality of the files used is excellent. People might say that my DAC/amp combo is not up to scratch, that might be so. I'm using the O2/ODAC combo which is supposed to be very transparent and uncoloured and I have read it should allow you to hear the true sound of the headphones. If this is true, the uncoloured sound of the T1 is too bright. The only flaw IMO but a major one for me since I am quite sensitive to the treble.
 
In terms of power, discussions in the sound science forums indicates that technically there doesn't seem to be much indication that the O2 amp cannot supply sufficient power to the T1 (http://www.head-fi.org/t/707169/can-the-o2-amp-fully-drive-the-beyerdynamic-t1). I'm sure that colouring the sound using a tube amp would be able to tame the highs and make these headphones truly fantastic but IMO the natural sound of these cans has some serious flaws in the treble. This seems to also be attested by the frequency response curves I've seen for these headphones and also numerous comments I've read stating the same point.
 
This being said, I really do like these headphones and aggressive use of EQ can tame the treble enough to make it enjoyable for watching movies and listening to music. Without applying any EQ it sounds truly superb for Classical and Jazz music (as long as there is minimal singing IMO). However on the whole, I will probably sell these and settle for something more bass/mid centric such as the HD 600/650 which are widely praised in combination with my current DAC/amp.

I've been listening to the HD 600 with the FiiO E07k/E09k combo. I realise that this DAC/amp combo might not really get the most out of the HD 600 but I thought I'd share my thoughts nonetheless.
 
Firstly, I really like the sound of the HD 600. It's detailed, the bass is tight and present but doesn't dominate. Personally I think these headphones sound great with rock and classical music, I especially find drums and violins to sound very realistic. The comfort of these headphones is superb, the ear pads are very comfortable and the headphone is rather lightweight. I think the fit is also perfect and ensures that these headphones don't wobble around much. I find the HD 600 comfortable enough for hours and hours of listening.
 
My biggest complaint about this headphone is soundstage. I've heard people referring to its soundstage as a 3 blob soundstage and I think that sums up my opinion perfectly. The bass while it sounds really good, it lacks the visceral component that adds emotion to some RnB, rap and pop songs. The HD 600 still sound great for these genres but I can't "feel" the music as much as I would like to.
 
Some other minor quibbles are that the headband does feel like it could break quite easily and I think there is slightly too much treble for my tastes. Movies sound realistic but do lack the excitement of bassier cans IMO.
 
Overall, I'd say that these are excellent headphones. Good all rounders that perform well with most (if not all) types of music and really shine when listening to certain genres. Comfort which is a vital requirement for me is also top notch!

These headphones are a huge step up from anything I've ever heard before. 
 
The only other well known headphones I've listened to extensively are the Sennheiser HD 202 and the Logitech UE6000.  Compared to these headphones, the sound quality of the X1 is better in every way. 
 
In my opinion the Fidelio X1 has a truly fantastic bass and treble response. The bass impact of these headphones during action sequences in movies really adds a level of immersion I've not experienced before. I'm also hearing a lot of details in music I previously did not know studio recordings picked up. Sometimes it feels like sensory overlord since I'm not used to listening to this level of detail.  I also find that the excellent bass and soundstage makes Jazz sound especially good with these cans. 
 
My only complaint with the X1 is that I sometimes feel the vocals in music and speech in movies are not as prominent as I would like. These headphones are also a bit bulk and slightly heavier than ideal in my opinion. However, the comfort of the ear pads are great.
 
I use these headphones with the Audioquest Dragonfly and I find this combination to be a great match. If you want to know which DAC/amp to use with the X1, check out the results of a  survey I conducted:
 
http://www.head-fi.org/t/689313/dac-amp-for-fidelio-x1-ratings-and-recommendations-compilation-thread
 
These results contains feedback and recommendations from over 30 Fidelio X1 users on the different amps and DACs they have used with this headphone.