I have been reading about the ATH-PRO700MK2 for awhile and have been curious to try them.  When I found one refurbished for $104, I decided to go for it.  I have been looking for a headphone with spectacular bass that will also serve as a great all-arounder.  While the PRO700MK2 isn't the all-arounder I hoped for, it is great at what it does which is produce powerful, deep, tight and punchy bass without bleeding into the mids.  It is a keeper that I will use with selective genres.
 
Comfort
 
Most of the shortcomings you have read about these is true, but to me, not as bad as described.  The pads are thin, but adequate.  The cups are too small for my ears which I consider to be average size.  The fit is tight, but I'm okay with that as I plan to wear these while exercising.  The overall build quality seems excellent- rugged and durable for active use.
 
Bass
 
There are many adjectives I could use.  I will simply say this is the most powerful good bass I have ever heard in a headphone, ever so slightly besting the M-Audio Q40 which was my previous bass king.  With the samples I have tried, the bass does not bleed into the mids which is very important to me.
 
Mids
 
The mids are decent- smooth and pleasing.  However, they are recessed and lacking detail, but have reasonable detail for their price range.
 
Highs
 
The high end is probably the biggest shortcoming of the 700 MK2.  They are slightly dark, but not as dark as other bassy "DJ" headphones I have tried such as some Pioneers and the Aiaiai TMA-1.  Highs are not as well extended as I would like and accentuate the lack of detail.  On a positive note, there is no sibilance as I have heard with the Q40, but the Q40 has superior extension and more accurate reproduction on the high end.  Another positive is that the PRO700MK2s are non-fatiguing.  If deciding between the PRO700MK2 and Q40, I will definitely take the PRO700MK2 after EQing.  See edit below.
 
Conclusion
 
While these can be adequately powered with a smartphone, they definitely benefit from (and can handle) an amplifier.  Since I primarily listen portably, I sampled with and without my FiiO E11.  Despite the fact that I don't consider the ATH-PRO700MK2 to be my "holy grail" headphone, I am pleased with them and will enjoy them when I listen to dance/club/hip hop.  My musical taste includes almost every genre and I will still be searching for a headphone that is more comfortable with highly detailed mids, well-extended highs that are neither bright nor dark and with the bass these offer which is perfection at a price I can afford (under $500).  I am holding out hope for new and upcoming releases from V-Moda and Denon.
 
Please note my value rating is based on my purchase price as refurbished.  New at $200 or so, I would rate value as a 3.  There is also an anniversary edition with gold accents available for about the same price new.
 
EDIT:  They are growing on me as my ears adjust to their signature, but I would still describe them as dark.  It just feels like something is missing from the high end.  After downloading an equalizer program (Denon's free music app), I was able to correct nearly all of my perceived shortcomings of these headphones.  With a tweaked U-curve, I was able to even further enhance the bass without becoming muddy or distorted and correct the darkness on the high end.  These are excellent cans with high quality drivers that are fully capable of responding to any input given.  While their detail is very good, if you are a stickler for detail, you will want to look further.  I consider these to be the best sounding "fun" can that I own.

I own a pair and love them.  I am surprised there is not more online buzz regarding these.  I mostly listen to music during activity but had difficulty finding a pair of active headphones that approached audiophile quality until I heard the C5.  The sound is clean, accurate and full.  The bass is prominent without being overwhelming which is perfect for workout tunes.  The sound is also enjoyable for home listening.  This is a great all around headphone for all types of music.  The ear loop also provides a secure fit and I sometimes forget they are on.  Related to fit, I have owned other earbuds that gradually slip out and the sound becomes hollow as the seal is lost.  I NEVER lose the seal with these because of the unique ear loop designed.  I was impressed with their sound quality from the first time I tried them on and haven't heard a better sounding headphone in their price range.