Pros: Glossy sound. Easy to drive. Extreme comfort.
Cons: Pushes size limit for a 'portable'. Soundstage a bit tight.
I was a happy P5 owner and I've been a huge B&W fan for quite a while. Their products always fall into that "Bose done right" category. B&W makes the consumer grade products which Bose SHOULD be making. Extreme high quality without sacrificing sound quality.
Enter the P7. I'm happy to report with the P7, B&W took everything they learned in the making of the P5, and improved upon it. The prime difference between the P5 and P7 is the P5 is on-ear, where the P7 is over-ear. The P5 were very picky when it came to ear placement. You could get 5 different sound signatures out of the P5 by simply changing how they sat on your ear. The P7 does away with all that fiddling. Put them on, and they're good to go.
But not right away. The P7 required significant break in before the bass came to life.
Out of the box I was immediately impressed by the build quality. This is another high-end assembly by B&W. The attention to detail is obvious. Highest quality leather ear cups and head pad. Beautiful chrome skeleton and brushed aluminum. These are remarkable headphones, and like the P5, they hug your head like pillows without any fatigue. They fold up nicely and come with a largish case. If you're looking for super-compact travel phones, these might not be for you. The other note on comfort and fit-- if you don't like phones touching your ears at all, these aren't for you. They are close on the ears. Sound isolation is excellent.
The sound quality is what I call glossy. Bright without sibilance, punchy without bloom or bloat. The sound is cohesive in the extreme-- very musical-- but there's not a lot of air between the instruments. The soundstage is decent for a closed headphone, but it's not of the HE-500 or HD-650 class a few hundred dollars up the road (and open as opposed to the closed P7 design). I like to call the sound 'winning' at first listen. It's an intimate appeal, I was drawn to the sound signature immediately and if you have nothing of a higher end to compare them to, you're likely to fall in love. Switching over to the HD-800 and you immediately hear the lack of air. Then again, the HD-800 is $1000 dollars more and is open backed.
The P7 is easy to drive. They can be used right out of a smartphone, but obviously benefit from decent amplification. My Pico Slim matches up very well with them and I imagine the Woo Firefly or even a Burson is probably the ultimate pairing for this headphone. My WA5LE was overkill.
I'm currently using the P7 at work, where I spend long hours wearing headphones. They've all but replaced the ATH-M50, which I rarely turn to anymore (at 1/3 the price). I've not done direct A/B comparisons with their nearest competitor, the Momentum, but I've heard both independently, and I favored the P7. I consider the P7 to be best in class at the consumer grade $400 price point.
If I were considering the Momentum, or a Bose active noise reduction headphone, I'd definitely steer you to the P7. You will be rewarded.