The isolation is surprisingly good with these headphones. My purchase was for office use because I can't hear a thing with my CIEMs (1964 Ears Triples) and my HD650s are too loud for those around me. I've already missed two phone calls so IMHO these are as isolated as you will get for over the ear cans w/out adding a noise canceling component. The Momentums are probably the most comfortable over the ear cans I have worn, even straight out of the box. They fit me perfectly, enclosing my ear without any pinching or overlap.
I agree mostly with Jude's review, however, I think the bass accented (not heavy as some have described) sound can be great if driven with the right component and listening to a music genre that can utilize the sound signature. I was listening to Foster the People "'Torches" and I actually prefer these cans to my HD650's for that specific genre (Rap/Hip Hop sounds great too). The HD650s being open have some significant advantages in regards to sound stage and overall balance so will remain my go-to for reference, that being said, the Momentums are flat out fun!
I did hear a significant difference when listening to these with my cowan D2+ vs. the ibasso dx100. The mic'd cord and marketing of these headphones suggest they are meant to be paired with phones and other mobile devices but I would suggest either an amp or high-end component (like the DX100) to get the most out of these headphones. I just didn't get the clarity and punch from my Cowan, in fact, I would prefer my CIEM's over the Momentums if I was just looking for a mobile headphone to pair with an on the go component like the Cowan D2+.
At $350 these might be a bit of an overkill for mobile headphones. You will definately be getting the most out of your source component (iphone, android phone, etc.) but the Momentums can do a lot more if you want to push them.