One can mix with headphones easily...
It is possible to produce great mixes on headphones, if you use GOOD headphones. Some are really flat and have speaker-like soundstaging. I'm not talking about the 7506 or low-end models here, which I would never mix with. I'm talking about open headphones that are diffuse-field equalized to sound like speaker monitors.
The Sennheiser HD600 are what I use for mixing and mastering.
The HD650 are even better. with more detail.
The BeyerDynamic DT880 ain't half bad, but the king of accuracy now seems to be the Sony SA5000. Those headphones are $250 and over, but are worth it.
If you really want to get monitor-like results, you can get a pair of electrostatic headphones, like Stax and the Sennheiser Orpheus. This is expensive stuff, but I guarantee you that this old "simili-fact" that one should not mix with headphones is only the product of people not using good enough headphones for the job.
Remember: A $500 headphone driven by an excellent headphone amp beats the crap of nearly any sub $10k monitor system. The detail is much better and the frequency response is much flatter.
People who say one can't mix on headphones are simply driven by old sayings or are so used to mixing on monitors that they can't translate well on headphones....or are simply using cheap closed headphones.
The argument about everyone hearing headphones differently applies to speakers too. Nobody hears things the same way. When I mix and master, I want things to sound good for ME, not for my bandmates or neighbourgs. Wheter I use flat monitor speakers with flat amps and perfect crossovers with a nice full extension sub OR quality headphones, what sounds good to me WILL sound good to me. No need for a room to be a "common denominator". Furthermore, quality headphones are flatter than anything else up to 20 times the price.
Having that said, I think mixing/mastering on cheap headphones IS crap, but once you get good/flat/accurate headphones and proper amplification, sky's the limit. If you've never heard a GREAT headphone mix, then it's probably because there is not a single GREAT engineer that uses headphones, because the mentality seems to be stuck with using monitors and this has been transmitted to generations of new engineers. Fact is, in 2005 we have quality headphones that rival the best monitor systems for much less $$$ and with less disturbance.
TheDewd