I purchased one from ebay, a Chinese version; thought I'd add my two cents. this one looks like it is missing the 3G 1700mHz frequency from Band 4 in MassDeduction's post. I'm in the NYC Metro area, the phone shows connection to T-Mobile 4G which I assume is LTE. The data speed is much slower than my other phones.
Speedtest.net shows:
Test 1 (Cell Data): 4.7 Down / 4.6 Up
Test 2 (Cell Data): 11 Down / 4.1 Up
Test 3 (Wifi): 16.7 Down / 10 Up
Ran the tests about 4:00PM EST. Generally my other phones are getting 15-20 Down and above 10 Up on cell data so it is disappointing. Currently the phone is running Win 8.1 as I was running Win10 TP but did a roll back; Win10 was running worse on this phone than on my 635 and that was barely usable. So I can live with the slower speed for now. Other than a T-Mobile version or a MS flagship phone, there really isn't anything else that I'd rather use. I have a Nexus 6 and it is disappointing, probably used 5-8 different Android phones prior to the N6.
Edit:
I put my sim back in my T-Mobile 635 and got the following speeds at 7:45PM:
10 Down / 10 Up
Switched the sim back to the 640 and the results are slightly slower:
7.5 Down / 9 Up
VoLTE doesn't work on the 640, the data icon switches to "H" as soon as you make a call.