I ordered the unlocked M8 yesterday through HTC, will be in my hands sometime today thanks to free overnight shipping. I had the same concerns as you. The rep at HTC said it is fully compatible with AT&T. From what I could find regarding LTE bands the difference between the unlocked model and the AT&T branded model is the unlocked version does not have band 7 while the AT&T model does. I couldn't find where band 7 was used by AT&T in the U.S., although it may be and I just couldn't find it. I currently use a Nexus 5 on AT&T and is has the same LTE bands as the unlocked M8 and it works fine on AT&T's LTE.