So how many of you guys giving advice on 'flat' headphones have done a lot of mixing/monitoring?
My advice: choose a pair of headphones with which you are very comfortable. You should know the signature of this headphone, know what certain recordings sound like, how these certain recordings also sound on other good/familiar systems (speakers, car, etc...), and how you like them to sound. Once you are completely familiar with this given headphone (assuming it doesn't sound like tin), you are ready to use it for monitoring and mixing purposes.
Lastly, use a comparison track that has the tonality and attributes you are looking for. Mix and monitor the new mix with the intention of trying to mimick some of those attributes. Eventually you won't need the comparison recording.
It doesn't matter how neutral your headphones are for mixing or even mastering!
This is largely misunderstood. Your ears and mind need to be trained.
Additionally, headphones tend to be good with applications such as editing, panning, eq'ing, etc... Speakers are good for checking the phase, soundstaging, depth of field, naturalness of tone, etc... Although I use headphones to make sure I haven't mixed a given recording with too much sibilance.
I find the HE90/HEV90 to be quite good as I am very familiar with its sound, and know what a good recording should sound like with this system. The ER4S is also a good choice. HD6X0 are used in recording studio's the world over...
...Having said all this, I will admit that it is here where clarity, transparency, etc... etc... the big 'audiophile' terms we speak of the most, can really help. However, like I stated before, get really really used to your headphones (and speakers) first.
I have never heard a headphone or speaker without some form of noticable coloration or 'signature'. Even those 'without a signature' have this as a signature... hear it!
Neil