We have a little shop in Vancouver called The Headphone Bar (he has an online shop as well with very competitive prices -
http://headphonebar.com). He has all of his models on display and ready to sound test until your heart's content.
I went in there last year and tried out a set of Sennheiser Momentum's over the ear models but, to make a long story short, ended up buying a set of Atomic Floyd Superdart ear buds (best sounding earbuds I've ever tried - even compared to the $1200 AKG set). Anyways, a year goes by and I can't seem to forget about the Momentums. I sold a bunch of gear and had some cash left over so I decided to take another shot at the Momentums.
Let me first say that they sound phenomenal. I tested them with a few different genres of music. I tried Colin James cover of "Into the Mystic" and Grace Potter's "Big White Gate". If I closed my eyes, it sounded like they were in the room playing right in front of me. It was ridiculous. I tried some dance music, hip hop and it handled everything very well. The bass was tight and deep and the highs were crisp and clear. Then I put on some Foo Fighters from Wasting Light, and while they still sounded awesome I found the highs got lost in the mix a bit. Just a bit. But enough that I could see myself tiring of it after a while.
I noticed a set of KEF M500's right beside the Momentums and decided to give those a try since they were in the same price range. These were on-ear as opposed to over-ear headphones. They sounded almost exactly like the Momentums but with the high just slightly rolled off. Just barely. They happened to be on sale for $259 vs the Momentums which were $299. So I picked up the KEFs.
The Momentums edge slightly past the KEFs for acoustic but the KEFs edge slightly past the Momentums with rock and metal.
So, IMO, you couldn't go wrong with either one.
I'll just add that the test was done with an iPhone 5 (iTunes standard 256k) straight into the headphone - no onboard eq tweaks or outboard headphone amplifier (although I do have one). And on that note, an $89 portable headphone amp can do wonders for a cheaper set of headphones. I was shocked.