My $0.02 on the 640. I couldn't wait, wanted to upgrade my Nokia 520. I have Android devices and was using an HTC One M7 Google Edition phone with Android 5.1 but the speakers on that phone, while good for music, are terrible for phone calls for some reason. After a couple of years, I never became comfortable with the "Wild West" approach of Android and the way everything doesn't ever seem to work smoothly together as a system.
I started using my wife's old Nokia 520 and really liked it. All I wanted was a phone with a screen as large as the entire face of the 520 and maybe a bit more RAM.
The 640 has 1MB of RAM, but to be honest the difference in speed is hard to notice between it and the 520. Some apps a tiny bit faster, some seem the same.
I like the 640 screen and Gorilla glass, but 2 problems bother me. First, there's a 3.5 mm wide black space on either side and a huge 15 mm black space on top or bottom. They couldn't use that for the screen?
On top of that, you have to slide up the home buttons, which then take more screen space.
So in the end, the screen doesn't seem as big as I thought and the black space all around really bugs me.
[ADDENDUM: I calculate the wasted space at 32%! The screen measures 69 x 138 mm, for 9,522 mm sq.
The vertical sides waste (3.5x138)x2=966 mm sq. and the top and bottom (15x69)x2=2070 mm sq. That's 3,036 mm sq. of wasted space, or (3036/9522)x100=31.88%.]
Other problem: the 640 is slippery as a bar of soap in a bathtub. I noticed this right away; as soon as I took the cover off to install the battery, I placed the cover on the side of the fabric chair and it slid right off. I'm going to look for some type of rubbery paint I can spray on the (orange) cover, this thing is way too slippery.
The cover seems cheap also and the speaker -- while better for phone calls than the HTC One M7 -- is pretty bad, especially if you lay the phone down while talking. Again, why didn't they use that black space for speakers top and bottom, on the front, like the HTC One?
The thinness of the 640 is good, but it just seems to me like there's a lot of wasted space here. I wish the phone was no larger than the visible screen real estate, that would be nice.
Otherwise, everything ok, once I figured out how to switch from Chinese to English on this eBay purchased (new) phone, I was able to transfer my old 520 settings in a jiffy, unlike the Android phones, which take me forever to re-download and install and set up all the apps.
UPDATE: I've been using the 640 for a few weeks now, I'm really liking it more. Couple of other things, the battery life is nearly unbelievably good. I can use the phone the same amount all day as I used to with the HTC One M7; the 640 still has 80-85% battery left at night where the HTC One I had to stop during the day to recharge it at least once! Amazing battery conservation on this thing.
The longer/taller size still bugs me a bit, especially when talking on the phone, if I have the speaker in my ear (and if you don't put it directly over your ear, the volume drops dramatically), the mic is too far away. One more reason that the blank black wasted space is ridiculous.
On the speakerphone, it's good but I do wish it had maybe a pair of better speakers.
Owning and using the 640 has got me thinking I may upgrade to a premium Lumia when the 840 or 940 or whatever comes out. I figure if I like the 640, then the 940 should be awesome!
Here's a pic showing the 640 vs. 520 vs. HTC One M7 and another showing the amount of black space around the screen on the 640.
