I got a 640 to replace my old (but very good 820) about three months ago. I have alot of respect for the 640 after using it for the past months. I also have a Moto G lte as my secondary phone (on Android of course) and I rate the 640 better than the Moto G at less than half the price. Performance wise it is about the same as the 820 (which has almost the same insides as the 920), but the battery lasts almost three days between charges, and it already has Denim which the older phones will never get, it also should get Win 10 if they ever release it. The camera is not as good; but it is not bad at all and I have no problem with it. While the 640 is on sale now I would jump on it if I were you. You can install about 90% of the available apps on the SD card (and they run fine on it) so 8GB storage is plenty.