I was looking to buy myself a nice christmas present so figured I wanted a pair of nice portable headphones. Up until now I always had sort of entry level headphones/in-ears, with the highest end being a pair of Shure SE215 IEMs, which I don't really like due to comfort issues. Anyway, I was first thinking about the momentum on-ears and really liked them when I listened to them in the local huge media outlet (Saturn, for those from Germany). Then I figured I'd also go have a look at the local Hi-fi audio store, which had the m500 on display. After listening to it and also to the B&W P5, I really, really liked the M500, way better than the P5. Also went back to listen to Momentum on-ears, which at that point were really uncomfortable compared to the M500.
So I ended up buying the M500 and man are they awesome! :) Since I'm green as grass here, I can't really compare it to other headphones in the same category, but holy s*** are they nicer sounding than my SE215s... All the music I listen to has an extra level of detail to it and songs I know well have all sorts of nice little sounds in them. :) Separate instruments have so much more space around them and they remain separate even when the music is "busy". The bass (for instance on Drake - Started from the bottom) is much more punchy without muddling all other sounds. Pianos (ie. Agnes Obel - Close Watch) have so much more detail and some chaotic, double bass heavy jazz (Phronesis - Abraham's new gift) still has a really nice crisp clear hi-hat in it. All of this from listening to it on my iPhone 4S, using Spotify in the highest quality.
As you can see I'm quite happy with my M500s, even though I might have been just has happy with any other headphone in this category (although the P5s are worse in my memory).
So far my 2 cents...