Canon isn't the first manufacturer to use a back-illuminated CMOS sensor; as usual that honour falls to Ricoh. In conventional sensors the wires connecting to the photocells run over the front of the chip, partly obscuring the light falling on it. In a back-illuminated sensor the wires come in from underneath, allowing more light through and improving low-light performance. The sensor has innately fast performance, which when combined with Canon's high-speed DIGIC 4 processor gives the 300 HS some impressive performance figures.
It starts up in approximately two seconds, and in single-shot mode it has a shot-to-shot time of approximately 2.3 seconds, which is actually not that fast, but it is the continuous shooting modes which excel. It can maintain 3.7 shots a second at full 10MP resolution, or 8.4 shots a second in the 2.5MP high-speed continuous mode. However it's worth noting that the Ricoh CX3 can beat it soundly on all counts and costs £80 less.
The main reason the single-shot shooting speed isn't as fast as expected is that the autofocus system isn't anywhere near as fast as I'd usually expect from a Canon camera. It takes almost a second to focus even in good light, and in low light it does appear to pause briefly before activating the AF assist lamp. Focusing is reliable and accurate though, even in low light, and the AF assist lamp has a range of several metres.
The f/2 lens does provide some advantage in low light shooting, but it's not as great as you might hope. It's only a one-stop advantage over the IXUS 110 IS, and it's only f/2 at the widest zoom setting. The aperture drops very quickly as soon as you zoom in, with a maximum of f/5.3 at full zoom, which is not much faster than most other comparable compacts.
The lens does produce good edge-to-edge sharpness though, with minimal wide-angle distortion. There is a little chromatic aberration visible at the corners of the frame, but it's not too obtrusive. The overall level of detail is very good for a 10MP compact, although it's not as good as Canon's own PowerShot S90. With a file size of around 3MB the compression is a bit higher than I would have expected, but overall image quality is very good, with superb colour rendition, flawless exposure and excellent dynamic range thanks to the advanced sensor design, especially with the i-Contrast feature enabled. Noise control is also exceptionally good, with virtually no visible noise at 800 ISO and usable images even at the maximum 3200 ISO, which is a remarkable performance for a small-sensor compact.
The IXUS 300 HS is a very good camera, with a nice easy-to use controls, a good range of features, decent performance and excellent image quality, especially at higher ISO settings. Build quality and handling are very good, and the design looks stylish and contemporary. However it doesn't offer any decisive advantage over cheaper products from rival manufacturers, or even some of Canon's own other models, and the ludicrously high price is a major disincentive. If it comes down to around £250 you can add another point to the overall score.