Here are some opinions about mixing with headphones.
For me, headphones are an ESSENTIAL part of the mixing process. Before I start mixing, I listen to each track individually (soloed) on phones. Often I'll catch something like really low-level mic handling noise or hiss that cuts in during a punch that I would never hear on speakers.
Also, it's important to consider the playback medium. More and more people are listening on earbuds and headphones these days, so you have to at least reference to headphones as much as you do to speakers, if not more.
It is entirely possible to make mixes on headphones that translate over any system. I was shocked when I was doing some video editing in Europe and all I had for monitoring was a set of Monster Turbine Copper earbuds. I did the mixes behind the video, fully expecting to tweak them considerably when I got back home. I put them up on the speakers, and didn't have to change a thing.
Speaker of airplanes - I never mix or master within 48 hours of being on a plane. I always end up having to redo them, so it's not worth it.
ATH-M50s are great in the bang for the buck category, probably the best out there at that price point. I also have a set of Beatz (don't laugh) for monitoring the lows, as most headphones are bass shy. I like Sony phones for DJing because the mids cut, which can really help with high ambient noise, but I never really warmed to them for mixing. AKG makes some great phones, they have the "airy highs" thing nailed. Ultrasones are fun for listening, but I've never been able to do mixes that translated well with them.