Yeah, what J. Smith said. They're listening headphones, and still a bit overpriced, even after taking that into consideration. For the love of God don't try to mix on those. Ever.
Here's what info I have to offer on headphones:
Brands like AKG will be much more accurate, however, that generally means they won't sound "good." For something like referencing a vocal mix,
K271 MK II's will be great. When you adjust some mid frequencies, for example, you can take off the 'phones and listen to your adjustments on a set of high-end studio monitors, and they'll sound the same. However, you'll barely hear the low end on the AKG's, I mean, it'll be there, but you'll almost need to listen for it to hear it. If you're going to be trying to mix beats with some heavy sub-bass (808s, or really any kind of deep bass/kick), the AKG's won't work for you.
*That being said, there are some K702's that I haven't ever listened to that could work for this. Maybe someone else here knows?* Also, listening to music for long hours does not go hand-in-hand with the bright midrange tonality of the K271s. They are really meant for reference and tracking.
All that being said, I can recommend to you the Audio-Technica ATH-50's. If you're producing, these will be great. They've got a darker sound to them. They're flat, except a huge drop in the mid frequencies. This is both a good thing and a bad thing. For an accurate mix, this won't really be helpful (most listening headphones have this curve in the mids as well; cutting the mids makes the sound less harsh). A vocal mix will probably sound completely different if you mix it and then you take them off. However, if you plan to have them on for several hours at a time, the smoother sound from the reduction of the mids will be a blessing to your ears. Aside from the mids, they have a fantastic frequency range, with highs that are clean but not painfully harsh, good lows, and then a sub-bass that is difficult to find in most headphones. Neither the low nor the high end are boosted, either.
Sorry if my writing is confusing or unclear... I'm exhausted right now and not necessarily thinking straight...