BAD! it would also be bad if you were mixing on new monitors for the first time, or monitors in new room or place. It really depends more on how complex and deep the material is you are going to be mixing and what purpose the mix is for. Headphones are very much different that speakers. Having said that, I have mixed many things on AKG 240 phones that I am very used too and got good results but it is not the same and would have been better if I had good monitors.