This review won't be too detailed, just a vote of respect for such a fine pair of headphones.  I tend to like warm toned headphones, rather than purely neutral sets, and the Philips Fidelio X2 has met my expectations in its ability to bring some bass into play, while keeping the airiness of open back headphones intact.  Sound stage is wide, but not deep with this set, and it has great energy/liveliness to the sound.
 
Though bass is its main claim to fame, the X2 doesn't seem to reach deep into the sub-bass regions, so for pure bass heads, it may not be enough.  Its been more than adequate for me, though the mids do recede too much sometimes and the highs are not always well controlled.  I find that mediocre recordings or low bitrate streams don't sound good with the X2.  Another characteristic of the set is its need for power.  Though they'll run from portable devices, or straight from the computer, they don't really shine until I hook them to an amp.
 
Physical construction is first rate, with excellent materials used throughout.  I'm not the greatest fan of suspension headbands, but these surprised me with both their comfort and effectiveness.  The pads on the X2 are large enough for my oversized ears and comfort levels are very good.  I can wear them for several hours at a time with no issues.
 
Overall, I'd certainly recommend them for casual listening, but they are not intended for those looking for ultimate precision and a razor edged analytical presentation. 

The NAD VISO HP50 could have been a truly outstanding headphone.  They produced a set with great sound, with a marvelous midrange, bass when it is called for in the music and highs that do not sound shrill, glaring or in any way harsh to the ear.  And for a closed back headphone, it has decent sound stage, though I don't think the "Room Sound" technology is superior to that of open back headphones I've tried in the same price range, or even some open backed headphones that are much less expensive.
 
If anyone has seen the review from the Wirecutter that said these headphones couldn't reproduce sound under 90hz, I can tell you that is a load of horse****.  Going to the online tone generator, I was able to hear tones from 90hz on down to 20hz, though with the 20hz tone, you feel it as much as hear it with these headphones.
 
No, the problems with the VISO HP50 is its physical design, and choice of materials.  The plastic cups look and feel cheap, and their glossy surface does nothing to dispel this impression.  The hinge on the cups has horizontal slop that I'm afraid will only get worse with use.  Though the headband adjustment stays in place where you put it, it also has slight popping or creaking noises that transmit directly into the cup and will annoy you if you move around very much. 
 
And now on to the pads.  There is no space within the pads for average to larger than average sized ears, so these headphones are more like an on-ear design for me.  The material used in the pads is soft and makes a good seal, there is very good isolation with these headphones, but the pads squash down with use and you'll be pressed against the material covering the driver in no time, since there isn't much space in there.
 
I left the worst part for the end, the look of the headphone.  While its on a stand, its tolerable.  While being worn, it looks ridiculous.  Truly, you look like you've attached a handle to the top of your head.
 
I have a difficult time recommending these to anyone, just because of the deficiencies in the build, but I have no issues with their sound, and use them when I want a more discerning presentation than my other sets.

This was the first headphone that I bought based on the overwhelmingly positive reviews from members here.  I was completely surprised at how much I disliked them when they arrived, and said so on the forums, where I was told to "burn them in".  I followed instructions on this and found there was virtually no change in their sound.  Muddy bass is the only way to describe their sound, and it overcame the mids on virtually all the music I tried them with.  And they introduced some distortion to highs that I couldn't reproduce using any other set of headphones I owned at the time.  It was a complete disaster musically, but taught me a valuable lesson about overly enthusiastic reviewers here at Headfi.
 
I wouldn't buy them again at any price, and certainly wouldn't recommend them for any use.