Managed to borrow a Lumia 640 for half a day yesterday, took it back and then bought the second hand Nokia 930...
The Lumia 640 is most definitely a fantastic device, very nice to hold and use, the screen is good, but viewing angles are poor, not that that's much of a problem, as normal viewing is fine, the quality is actually pretty good, nice to have a SD card slot and yes as soon as I put the 32GB card in it immediately sorted everything out and just worked, the whole system is slick and very fast for a ?120 device, incredible value....BUT
I just love the Nokia 930, the screen seriously is to die for, it has to be the greatest screen ive used on any device I've ever owned, that amazing colour vibrant amoled, just cant be beaten, the performance, although the 640 is fast, the 930 is instantaneous, ballistic, most of the time faster than my brain can catch up, and i love this super quick, makes the phone seem miles above any other flagship ive used, even the iphone 6 seems slow. The camera, again the 640 does a pretty good job, but the Nokia 930 just seems more like a SLR camera than a smartphone, so I just couldn't resist going for the L930.