i do everything in a pair of akg monitoring headphones, personally. i dont even have monitors, and have never had monitors. my other reference points are a cheap kustom pa system and my crappy car stereo.
my mixing is solid, workable but obviously not great. my mixes tend to sound good to very good in higher end stereo systems and home theater systems so i guess i am doing something right.
honestly though monitors are great and i will have some at some point but even then you still need to do the car/ home stereo/discman/pa system/home theater/etc tests as well and even if you dont have great monitors or monitors at all you can still get good results!!
a headhone mix is great to me because it allows me to focus on panning and working things out in terms of space and placement, automation, as well as effects..... and i think headphones arenecessary for this, leveling i personally think that there needs to be a happy medium between the headphone mix and the monitor mix- but if you want a dynamic mix with motion and space then headphones are a great part of that.
seriously, i know there are "rules" and whatnot but i am breaking them all and getting solid results, therefore to me its all about your own ears and developing your own style with WHAT YOU HAVE that will lead you to your best mixing work!