In terms of performance the FS10 is, not too surprisingly, quite impressive. It starts up in a little over two seconds, and shuts down again in about one and a half. In single-shot mode it can take a picture every 1.2 seconds, which is nice and fast. I've already mentioned the continuous shooting mode, which at 30fps and full resolution is amazingly fast for such a small camera.
The autofocus system is very fast and accurate, as Casio's usually are, and even works well in low light, thanks to a nice bright AF assist lamp. Oddly it seems to operate even faster at full zoom than it does at wide angle. The close -focusing macro range is a bit distant at 10cm, but on the whole focusing is quick and reliable. Unfortunately the low light performance is slightly let down by the flash, which is underpowered with a maximum range of less than three metres, although it is well metered for close range subjects and avoids blowing out highlights.
When it comes to image quality I'm afraid the news isn't good. Like the EX-FC100 it suffers in comparison to other £300 cameras, but to be honest the FS10 looks weak compared to cameras costing less than half as much. It's hard to say exactly what the problem is; the lens appears to be good, with minimal barrel distortion, good edge-to-edge sharpness and almost no chromatic aberration, but for some reason shots just never look sharp or well detailed. Maybe it's the noise control system, which is very strong and obtrusive, blurring out detail even at the lowest ISO settings. Colour rendition is also a bit pale, and dynamic range is extremely limited, with very dense detailless shadows in high-contrast shots.
The Casio Exilim EX-FS20 is an interesting and technologically impressive camera with some almost unique abilities. It is well made, attractively designed and even handles reasonably well, but unless you have a pressing need for its high-speed abilities it is actually a fairly mediocre ultra-compact which is outclassed by cameras costing half as much.