I'm relatively new to mixing and made the initial mistake of trusting my headphones-only for mixing. The downfall of headphone-only mixing became apparent when listening to my mix on an iPhone, car stereo, Hi-Fi HT system, laptop speakers, etc.. I still haven't gotten around to acquiring monitors, nor is my space acoustically treated. At this point, as long as I understand the limitations of the cans and have a strategy to compensate, I'm fine doing a lot of my initial mix on headphones. You should listen to your final mix on a variety of sound sources anyways-- not just monitors.