Tech Preview seems good on my 640. The new Microsoft apps are cool, but I am still getting used to them (I came back to the Windows Phone because of the News/Sports app and Here Maps basically).
2 problems:
yesterday I didnt have Data connection at all. I went to Settings > Network > Cellular and Sim and it showed nothing, only the link to 'Learn More'. I came here to complain about it and now its working (I rebooted it earlier today, that probably solved the problem).
So yesterday I was all day without Data connection (problem above), WiFi on all the time (even when I am not at home), didn't use it much, just occasional Whatsapp messages. By the end of the day I had 70% battery. Impressive, even though without Data.
Another thing, Cortana doesnt work. I press the search button, it goes to cortana, wants me to go to Settings, I select it, it seems to load some new page and fails then go back to the same page. Beh.