As I understand the Mix was not designed for use in the US and it's missing some of the US LTE bands. You WILL get some though, for example on T-Mobile LTE bands are 2, 4 and 12 with 4 and 12 being the primary bands, and you DO get band 2 and 4 on the Mix, BUT not 12 (12 is the best to have, BUT it's not the end of the world). On AT&T you get 2, 4 and 5, BUT not their primary band 17. You get 2 and 4 on Verizon, but not their primary 13 AND no CDMA backup, so I wouldn't trust it on Verizon. And you get nothing on Sprint, plus their is NO way Sprint would activate it anyways.
I will tell you this despite that I almost got one. I'm on T-Mobile, JUST for that gorgeous screen!