Originally Posted by bennyboy
There's no such thing as burn in.
That is my opinion on the matter too, from a technical standpoint. I believe though that there are psychological factors at play - one's ears and brain have to get accustomed to a new sound signature, I don't think one can thoroughly evaluate a new (to oneself) signature right away. For me it takes at least a few days before I could say what I specifically like/dislike about a pair of phones.
Of course if you hate a certain sound signature right away (like it seems you're not a fan of bass heavy sound to begin with), chances are slim that it will grow on you and become your new favorite. But I was already in for a few surprises along the way (hated the SM3 at first, now love them - loved the IE8 at first, now find them not that super-awesome anymore, etc).