Okay, so I posted my full review:
Here are some additional notes to go along with the review.
I think the sticking point for a lot of people is going to be the bass. I did not observe any changes in the bass with burn in. If anything, I kind of liked the On-Ear less the more I compared it with other headphones in its class. (V-Moda M80, Sennheiser Amperior, even the lowly 'solid bass' WS55X). The bass response kicks in very strongly. On some tracks, it is fun and exciting. On other tracks it sounds like someone added the sound of listening to a car stereo from outside the car - a low drone that clouds up the music.
In reference to the above comments in the thread, I do not think that Sennheiser is producing inconsistent batches of these headphones. This is Sennheiser we are talking about, not a boutique audio company. But I do think that the bass is better or worse depending on what kind of music you listen to.
On tracks where bass is an 'effect' - mostly with machine drums or synths in more 'dancey' music, I think the On-Ear's bass is quite welcome and fun. On tracks where the bass is just meant to be another instrument in the mix, it sounds fairly unbalanced. Which kinds of tracks where the bass is what is in the end a subjective thing.
For instance, on this track:
The synth drum is meant to give the music an insistent, itchy quality. On the Amperior and the M80, it makes its presence felt without drawing too much attention. Listening to this on the On-Ear, the drum really starts to kick you in the ears. It's your call if that is a thing you like. Personally I think it is too extreme.
This isn't to say that the Momentum On-Ear is a terrible headphone. I think it does its chosen signature quite well and sure as heck sounds better than on-ear headphones like the UE4000 or Skullcandy Navigator. If you want a really strong V out of your headphones, this is probably going to make you happy. I personally could not really recommend anyone buy this headphone without listening to it, but if you love the design (and it is truly a nice, well thought out design) or you are buying it for someone who is not going to spend time comparing against other great headphones in this category - DT1350, Sennheiser Amperior, V-Moda M80, Audio Technica ES700 - I can't imagine they would be devastated.
For everyone who was kind of hoping for cheaper mini-Momentums in terms of sound, these aren't the on-ears you're looking for.