The 870's overall performance meets Canon's usual high standard. It has an impressively fast start-up time of just over one second, and a shot-to-shot time of 1.8 seconds, while in continuous mode it can manage a shot every 0.7 seconds without any buffering problems. The autofocus system is exceptionally fast and accurate, and low light performance is also impressive, focusing quickly and reliable in any lighting conditions including total darkness thanks to a good AF assist lamp. The flash is a little underpowered however, and doesn't really fill the frame at wide angle, leaving the corners looking a bit dark. Canon claims a wide-angle flash range of 4.2m, but this is a bit optimistic. The flash is well metered though, and doesn't overpower close-range subjects.
You can usually rely on Canon cameras to produce very good picture quality, and the 870 IS is no exception. The level of detail is superb for a 10MP camera, and the relatively low compression at the highest quality setting ensures that none of it is lost. The lens also performs superbly, with excellent centre sharpness and only a tiny amount of blurring and chromatic aberration in the corners of the frame. Wide-angle barrel distortion is kept to an absolute minimum, but this is most likely aided by distortion correction during processing.
Colour reproduction and white balance are excellent, although dynamic range is a bit limited. The iContrast feature, which is supposed to enhance high-contrast shots, really only helps with shadow details and does little to prevent blown-out highlights in awkward lighting conditions.
Image noise handling is another Canon strong point, and the 870 IS produces excellent results. Shots at 80-200ISO are largely noise-free, and the noise reduction at 400 and 800 ISO produces a pleasant effect that resembles film grain. It has maximum ISO setting of 1600, rather than the useless 3200 ISO features on many of its rivals, and shots at this setting may be a little noisy but are quite printable.
The Canon IXUS 870 IS is a superbly well made and very attractive luxury ultra-compact with excellent performance, a useful zoom range, an outstandingly good LCD monitor and superb image quality. It's a little light on features, but the ones that it does have are useful and work well. The only downsides are its slightly awkward handling, an underpowered flash and the relatively high price.