It hasn't been too bad at all for me when I've used it. The 640 is one of my backup devices, though, so someone else on here would need to comment on its daily driver ability for this current build (note: a new build is expected soon-ish).
I wouldn't do it yet... It loads slow and is not for daily use at all... The new update is supposed to be leaps better... I would wait... Shouldn't be long now
I've got mine for more than 2 weeks now and I didn't even think of getting the preview cause as far as I can see it's still pretty much unstable, and this is my only phone at the moment and I'm not willing to take the risk.