For me, they checked a lot of boxes and they will get a lot of use until I decide to spend a lot more money on another pair of closed headphones.
1. They are very travel friendly because they seem to be put together with the thought that you will just chunk them in a bag. The hard shell and the pleather (which doesn't attract lint like the cushies on a pair of Grados) are perfect for travel.
2. Closed with decent sound quality - I prefer K701 headphones, but I can't use the K701s in the same room as someone watching TV because they leak in and out too much noise. Therefore, the M50s get a lot of use. They do not have the huge sound stage of the K701s and I do not believe they are as accurate. However, I do think they sound good with every type of music. The Grado SR60s, to my ears, have a bright signature that can be annoying with some types of music. (I do not own any of the higher end Grados, but from what I have read, the signature is similar up the line)
3. Sound descent plugged directly into an iPod...so do the Sr60s, but like I mentioned, I prefer the sound of the M50s overall.
There are tons of posts about them on Head Fi and many people prefer them to more expensive closed phones. I will probably get some of the higher end Shure closed phones down the road because they are supposed to be similar in sound quality to my K701. However, having the M50s in my collection makes that less necessary, so I am not in a big hurry to shell out the money.
I would agree that with certain music they can be tinny, but overall, I am pleased with them.