Excerpt: While the description says over-ear, these are definitely on-ear headphones. Of the many (~10) sets of on-ear headphones that I tried, these were far and away the most comfortable. The sound has improved as they have burned in (I listen to a wide variety of music), though the sound certainly won't blow anyone away.
Pros: Comfortable, Compact, Great Sound, Lightweight, Stylish
Summary: KEF are a speaker manufacturer of the old heritage, stuffy hi-fi systems with mortgage level speakers. As has been the case with many audio manufacturers, the headphone sector is where it is hot right now and everyone (KEF included) are wanting a piece of the pie.
This is KEF's first stab at a headphone, and very good it is too.
Pros: Amazing detail retrieval, decent soundstage for a small can, high end engineering and construction
Cons: Slightly dubious fit, can be a little wobbly on the head when walking around
Until now, my pick for the best sounding portable headphone has been the Sennheiser Momentum. While slightly mellow overall with a noticeably forward bass and laid-back treble, until now the Momentum has provided a naturalness and transparency unrivaled by any competitor I've had the chance to try out.
Pros: Fantastic sound. Amazing value. Premium build quality. Very comfortable for an on-ear headphone. Very portable.
Summary: The search is over
For a long time I've been looking for a foldable closed headphones that sounds fantastic and is easy to carry around for my travels.
The KEF M500 has everything I need. First of all, it sounds great in all genres. I've tried pop, rock, classic, jazz, heavy metal, opera, and it handles everything well.
Pros: Excellent sound in all genres, very comfortable even with glasses, very efficient (no need for amp), build quality, foldable with hardcase
Cons: Slippery headband (not suitable for other active use than walking), isolation is a little below average (leakage is low though)
Summary: I will try not to repeat too many of the comments others have made, but I guess people may want my thoughts on using these headphones so here goes after two months of ownership. Comfort. You need to position the phones correctly to ensure comfort over a 2-3 hour listening session, otherwise the pads seem to press against prominent parts of the ear and cause discomfort almost like a burning effect.