I've been using a pair of the non-iPhone equivalents of these headphones for the past 3 years. The sound quality of those is, in my opinion, hard to beat without spending silly money, and they've only just begun to disintegrate after 3 years of daily use.
On first plugging the MM50s into my iPhone, I thought there was something wrong. The sound was clear, but lacked the weighty thump I was used to with my old Sennheisers and seemed unbalanced. Then it occurred to me that the plugs didn't seem to fit my ears as snugly as my old pair, so I put the (included) larger size plugs on and, immediately, I heard what I expected: crisp clarity, wonderful tight lows, and a kind of spacious, uncluttered sound. Who'd think that the plug size could make so much difference?
I find the plugs very comfortable, but in-ear headphones are a love/hate thing: your mileage may vary. The built in microphone seems fine, and the play/pause button is a great feature since one big drawback with the iPhone over a classic iPod is that it's much harder to operate it while in your pocket.
Based on my past experience with my old Sennheisers, I'm expecting that these will be plenty durable for daily use, and my one niggle with the old ones - an annoying rubbery texture to the cable, which easily tangled and attracted dirt like a magnet - is absent on the MM50s.
Overall, I struggle to fault these headphones, and especially for the price (which is not much more than a pair of standard Apple 'phones, which are awful). I highly recommend taking a moment to find the rubber plugs that fit your ears best (there a 3 sizes included): getting the right ft really does make a world of difference to the sound quality, as well as comfort.