This is my first official review for any headphone on Head-fi, so I will give it my best shot. Anything you want clarification on, please let me know.
 
Previous headphones: B&W P7, Sennheiser Momentum Over-Ear, V-Moda M100, Denon AH-D2000, Denon AH-D1000, Denon AH-D1100, Bose Quiet Comfort & Audio Technica ATH-A700X.
 
Current headphones: Denon AH-MM400, Marshall Monitor
 
Sources: Fiio X5, Sony Xperia Z2, Roland Quad-Capture, Lossless music formats, mp3's.
 
Usage so far: I have had about 250 hours of listening to these.
 
First impressions: Nothing short of gob-smacked!
 
Packaging: Understated, simple and beautifully packaged. What you are greeted with once the box is open is a beautiful piece of audio equipment.
 
Size: While people are calling these portable for their ability to fold, I would not consider them small or unsubstantial. Bigger than the original Momentum Over-Ears, marginally smaller than the B&W P7.
 
Portability: You could take them on the road without issues given their size and foldabilty. I personally would not take them out of the house as I feel they are too premium.
 
How they look on: The headphone is not too large or bulky to take on the road. However I do feel their headband does not closely follow the shape of your head, tending to stick out and look a bit square-shaped when on. I suspect this would still not deter most people from making them their mobile cans should they choose to. 
 
Build quality: This is a beautifully built headphone. You can tell you are holding something premium the second you open the box. Their finish is of a high standard, from the wood to the metal parts. One of the nicest pieces of Audio hear I have personally laid my eyes on.
 
Cons: The earpads are not leather from what I have read (even though as soft as you can get). The ear pads do not seem to be removable/replaceable (based on my simple test of trying to take them off). The 2 cables provided could have been thicker, sturdier & longer for a headphone of this price. Some on the main MM400 thread have reported cable noise. The cable connection into the headphone seems to be slightly oval in shape, so a 3rd party cable that is compatible could be hard to come by. There is a rattling sound coming from the right side, as pointed out by others in the main MM400 thread. I noticed it too only after switching my music off and moving my head around. It seems to be coming from the hinge that allows the arm to fold. That is where the negative attributes finish fortunately.
 
Value: Definitely worth their price of AUD$525, what I paid for them to get them imported and delivered from Japan to Australia. Look up cyrusgod on here or eBay if you're interested in purchasing one for yourself. I wouldn't hesitate recommending him to anyone.
 
Comfort: As comofrtable as headphones get, however given the number of Denons I have previously owned, I am not surprised. To draw a comparison, I sold off my B&W P7 and V-Moda M100 because of comfort issues. The P7's created a hot spot on top of my head. The M100's created a hot spot on my ears. 
 
Leakage: Had a colleague and family member throw them on. I put the volume up to a point I would find them too loud to listen to myself (i.e. level 90 on my X5). I could not hear a thing, so there is no sound leakage to be found. Without any music playing, you can hear enough to know what is going on around you. However with music playing, they effectively drown out the outside world.
 
Overall Sound: They are what I wished the P7's were. They have no harshness in the top end. They have no bloating or distortion in the low end. They are just effortless to listen to. Their sound could be best described as buttery smooth.
 
Soundstage: Is reflective of the source and track quality. I initially thought it wasn't large but as I dug deeper, I found it as good as the P7s.
 
Instrument placement: Once again, is reflective of the source and track quality and is on par with the P7.
 
Instrument separation: Same as above. The more I break them in, they exponentially seem to be improving at this. I had a 3 hour uninterrupted listening session last night and I was nothing short of mesmerised by how good this was and how they never sounded confused with any track I threw at them.
 
Top end: Stunning. Fully present, never veiled and without ever sounding harsh. Of all the headphones I have owned, this is as good as the top end gets.
 
Midrange: Slightly recessed, not as forward as the Momentum Over-Ear. Buttery smooth, articulate and improving as I keep breaking them in.
 
Bass: Can tend to sound a bit bass heavy out of the box with the X5, however the quality of the bass is as good as it gets; fast acting, not bloated or distorted. Also note, the amount of bass present was spot on with the Z2 and Roland Quad-Capture, so it seems to be more a case of the X5 having a warm presentation rather than the headphones themselves. Definitely better bass quality than both the P7s and Momentums. Is present and has impact when needed. Does not make an appearance when it shouldn't. Is flattening out as I listen to them more.
 
Other thoughts regarding sound: Definitely does not sound anemic but full and enjoyable with all the genres I tried so far.These things really come to life with lossless formats. I also noticed something with these I haven't heard before with my previous headphones. With other headphones, there tends to be a predictable sound irrespective of what track you threw at them. With these, each track has its needed character. It strongly reflects the quality of the recording, the emphasis the recording has with different instruments and the way in which it was recorded. For example, I listened to Sting and the Eagles (Hell Freezes Over live). I have both in lossless. For each artist, I had studio and live recordings. I had recordings which were older versus newer. In every instance, I got something unique and reflective of what I was listening to. The old recordings showed their age and imperfections. The new ones showed some of the artificial effects you tend to find with them. The studio recordings felt they were recorded right there. Live recordings displayed the feeling of a full stage with instruments placement quite surprising for closed cans. My previous headphones, particularly my Denons, tended to sound more predictable across the board. They also artificially created reverb to make themselves sound more open when in fact they just sounded confused. 
 
Final thoughts: The most enjoyable and intriguing headphones I have owned and listened to. Shame about their name as their persistence with the "Music Maniac" line attempts to make them sound youthful in order to go head to head with the likes of Beats. They are far from it. They have basically taken all the things I have disliked with all my prior headphones, fixed them and thrown them into one neat package.

EDIT 1: Came across a review with a frequency response curve for the MM400. While I don't know how to interpret these graphs myself, everyone on head-fi is saying it does not represent the sound signature. Either way, thought I would share:

http://www.digitalversus.com/headphones/denon-ah-mm400-p24663/test.html

EDIT 2: So I took the plunge and bought the 1.2m version of the NewFantasia cable below:

http://pages.ebay.com/link/?nav=item.view&id=311061670498&alt=web

I will start off with all the good news. It fits perfectly without any alterations. It looks stunning. It does not change the sound relative to the original cables. It is much higher quality and better built than the pictures suggest. My beloved headphones now look like they're worth a grand with the modified cable. Now to the bad news, there is still cable noise when it rubs against your body. In my case, I only ever notice it with the music completely off, so this is not a deal breaker for me personally.