Pros: Very comfortable for long listening in your lounge chair, plane, train or bus. Handles most genres without fatigue. And sound rates a 8.5-9/10 for me.
Cons: None really. Not cheap but hardly expensive compared to many other commercially successful phones like Beats, Amperiors or Momentums.
I tried many portables over the past 6 months looking for something I could wear pretty much anywhere and easily driven by my phone, iPad and Sony WMZ Z-1070. First were a set of DT-1350s. There are any number of glowing reviews. I must say I didn't mind the sound but I couldn't get a good seal and therefore the bass was underwhelming while the mids were too forward. I tried to like them but had to give up and decided to move them on recently. I also tried Amperiors early on but while some say the P5 has a forward mid bass, I found the AmperIor much more so. To the point of being boomy to me. I returned them after a week. That's when I got the P5s. I've enjoyed them since. Used them at home and while travelling. While I do prefer my Bose QC-15 for noise cancelling on flights, I just don't like the Bose for regular listening. Not that they sound awful, just not good enough.
Believing there might be better choices, went on to buy a set of Momentums after auditioning a pair and getting a great deal on them. It was a love/hate thing for me. They looked great, have the iPhone controls, a nice carry case, nice earpads but in the end I found them just too warm sounding, and I could not wear them for long periods as the stitching around the inner pad irritated my ears. I persevered for a month or so then bought the HD25s which I should have bought right at the beginning but chose the DT-1350. They sounded just right to me. So I traded the Momentums for my HD600s with someone looking for a portable and not using the 600s.
The HD25s really are a superb sounding phone but, they look too dorky to wear as an out and about portable and really aren't anywhere near as comfortable as the P5s. So what's the attraction to the P5? Simply the fact they do what a portable headphone should do well. They sound good even if not audiophile standard, are compact (they fit nicely in my computer bag), are very comfy for long periods, and totally non-fatiguing. I frankly enjoy using them. Vocals tracks present like you're in the audience not simply listening to recorded music and that perception extends to jazz and classical alike. I highly recommend them.
PS. I like my P5s so much after auditioning some P3s I found an almost new pair on eBay at a fair price purchasing them as a carry round pair of even more portable headphones. Make no mistake, while pretty good too, they aren't in the same league as the P5 and aren't that much more portable given they don't fold flat like the P5s do. Nor are they as comfy as the cloth pads don't sit on your ears as well as the leather pads of the P5s.