I agree with hekeli. I've never had a headphone that changed enough after the first hour or two to change my opinion of them and I don't think its wise to recommend people give a headphone 100s of hours of burn-in before deciding if they like it. In most cases you'll know in the first day whether you like the signature or not and you shouldn't be expecting that to change. Preferences do change, and "getting used to" a particular headphone or sound signature does happen, but headphones themselves don't change. Don't hold your breath waiting for them to change when you could be enjoying something you actually like.
It seems like this has been turned around on hekeli and I don't think it makes a lot of sense. What's all this about he shouldn't expect a new headphone to be good for the studio? He has every right to expect a $280 headphone from Sennheiser that's specifically marketed as a studio headphone to be good for studio use. And he has every right to complain if it isn't. We've been waiting all this time for HD6 impressions and now that we've got some we're going to blow them off if they aren't positive? Bleh.