Nikon's advertising for the S640 claims that it performs as fast as a digital SLR, with a start-up time of 0.7 seconds. I'm not sure exactly what they're supposed to be measuring there, but the actual time from pressing the power button to taking the first picture is approximately 1.2 seconds, which is still plenty fast enough and much quicker than most of its rivals, although it does take somewhat longer to finish booting the menu system. The autofocus system is exceptionally fast, indeed almost as fast as that of a DSLR, but the shot-to-shot time of approximately 2.1 seconds in single shot and 1.2 seconds in continuous mode, while reasonably quick by compact standards, isn't even close to the shooting speed of even an entry-level DSLR. So much for advertising then.
Overall image quality is very good. The lens is nice and sharp, and produces virtually no wide angle distortion, although there is a touch of chromatic aberration at the corners of the frame. Colour reproduction is excellent, with well saturated colours and a natural colour balance. Exposure and focusing are reliably accurate, and the camera performs well in low light conditions. Low light focusing is especially good, focusing quickly in total darkness at a range of several metres thanks to a nice bright AF assist lamp.
Nikon claims that the built-in flash has a range of an astonishing eight metres at wide angle in auto ISO mode. I tried it outdoors, and if you're prepard to put up with the image quality at 3200 ISO then yes, it will just about reach eight metres. If on the other hand you set the auto ISO range to a more sensible 100-400 ISO the flash range is a much more realistic four metres, which is still pretty good..
Image noise is also very well handled. Noise effects are visible from 400 ISO upward, but images still have good colour, smooth gradients and plenty of detail even at 1600 ISO. The higher settings of 3200 and 6400 ISO are less good, but still far from hopeless at least for smaller images.
Although it doesn't live up to Nikon's extravagent marketing claims, the CoolPix S640 is still a very competent camera. Build quality is good, it handles well, performs briskly and takes a decent picture in a wide range of situations; low light performance is especially good. It's not quite a bargain, but it is a nice little camera.