Well, I've used my M50s in the way that you've described for years and I'm quite satisfied. Isolation is quite good. Good bang for the buck and pretty versatile for different musical styles. Very good build -- which counts for a lot dragging them back and forth to work every day.
I find mine comfortable, but not in the same league as the Beyers, Senns, and big AKGs. Naturally some of this will depend on the individual fit. My ears are such that the m50s don't touch the pinna -- but I've read other postings where listeners report that the cups are not big enough for them and exert too much pressure. Also it seems that listeners who want strong bass tend to be disappointed; you can find plenty of postings on this.
For airplane travel I think you'd find that IEMs offer even better isolation. I sometimes take my Etymotic MC3s to really close out the sound of the jet engines. Those have a pure, clear sound (but a bit light on bass). However, I can't wear them quite as long as the ATH M50s.
Moving up the Sennheiser line to the HD 439 will give you a lot more comfort with velvet ear cushions and light weight plastic construction. Not as clear and extended and the ATH M50, not as durable, and not as much isolation. Still I find them very comfortable... and they do have detachable cable, so that can be replaced if needed. I got mine (for a temporary work situation) from a local best buy for about $80. Does a nice job of blending bass to mids and conveys a better sense of 'space' compared to the M50s. Still it's a closed headphone and not as spacious as most open backed models.
