I wouldn't use them for mixing anything, but in the mobile category, I love my Bose MIE2i's. They sound decent and they're not full in-ear which I hate. In-ears never work for me when exercising and the Bose stay positioned decently, have good bass response and generally sound much better than any Ipod type solution I've used before. Stuff on my Ipod has never sounded so good. I can even hear the bass player and hold a conversation at the same time if I want. I've thought about trying them as monitors, but haven't done so yet.
I did look into and eventually purchase the Audeze headphones based on Luke's ongoing recommendation. They are awesome, though I find that if I tweak only with them, I tend to make patches a bit bright because they have a little high end roll off.