How do they sound compared to the Momentums?

In my ears they sound better, specially the treble is more refined detailed and smooth compared to the momentum, and the comfort is much better. The momentum does not fit well on my ears, which makes them uncomfortable after just a short while.

How is the soundstage and imaging? And is it as clean sounding as the momentums in the mids?
Yeah the momentum are for people with smaller and average size ears so if you are above average in the ear department no dice for you.

Very good soundstage, definately among the best I've heard in closed headphones, mids are clean, it's a neutral performer. The pressure of the momentum is just too much compared to the M500, there are no comparison regarding comfort.

Pressure is a double edged sword. I think the Momentum strikes a good balance between comfort and staying put snugly on the head when moving about. You also get better isolation with a little clamping, and nothing beats isolation when you want to improve SQ in noisy environment.
I would however use IEM's instead of closed backs in such circumstances.

Well, headphone comfort is highly subjective, if you like the momentum, do keep them, for me M500 is a much better choice.

Thank you for the review. Are the M500 earpads and headband covered in leather or in a synthetic material?

It really looks and feels like leather, but from the smell of it, I think it's synthetic.

Hope you won't mind a noob review but I've just bought these cans pretty much on an impulse having finally had enough of a long line of very good in ear buds (Shure EC3, Klipsch Image S4i and Ultimate Ears 700's). I had shortlisted Scoshe RH1060, UE 9000, Parrot Ziks etc to look but then saw these at a local KEF dealer, tried them and I have to say I'm astonished at how good these are. At first use the mids and highs did seem a little "muffled" by comparison to my last "in ears" but after a few days use the mids and highs seem to have become opened up, although that might be just because I'm becoming used to the sound rather than the cans running in. The low frequencies are on another level compared to the in ears (not surprisingly perhaps) and altogether the M500's produce an extremely balanced sound with tremendous sonic and spatial detail. As for comfort - I've had these on for over two hours wearing glasses (and living in Hong Kong I'm very pleased that sweating is not a major issue) and have so far not suffered any discomfort. The noise isolation is more than adequate given they are not noise cancelling and extraneous noise - who cares!! To sum up I have to say this is the without any doubt the best money I've spent on audio for a long time.

Would you consider them sealed enough that others can't hear leakage, especially at night in bed when ambient noise is low?

Hard to say. Wore them in a lift, which is about as quiet as you can get in a public place, and no one was staring at me in disgust, and in Hong Kong they would!!
The only criticism I would make is with the cable (but not the fault of KEF) - I bent down to pick something off a low shelf and the rubber type cable caught over my knee so that when I stood up the cans got ripped off my head because the rubber coating is very grippy! And the length of the cable makes it easy to snag on passers by or other things so care is needed when wearing them especially if you're short!!

What Hi-Fi gave these cans 4 stars but gave the Beyer Dynamic T50P 5 stars. I've just listened to the BD DT1350 pro monitors (very similar to the T50's but a much wider frequency response) side by side with the KEF's (which I've had for about three weeks now) and the difference was very noticeable - I wouldn't exchange the KEF's for anything!!

How do they compare to k267? is there a big difference between those? coz I just bought the k267 about a month ago, the soundstage of those is not really that satisfying to me. I feels "Welcome to Head-Fi, sorry about your wallet" is already happening to me XD