While the S90 doesn't have the image quality of true single-lens reflex cameras, it packs an above-average size sensor that's closer to those found on full-size rigs. It also has a fast f/2.0 lens that lets twice as much light in as most compact cameras. The larger sensor and wider (faster) lens provide greater image quality and better shots under low light conditions. But I did not decide on this camera just for its sensor and lens. What sets the S90 apart from other point-and-shoots is a fully programmable control ring on the front of the camera that allows the user to adjust shutter speed, aperture, ISO, focus, zoom, exposure, or white-balance.
The selection of the front ring's function determines the setting of a second control wheel located on the back of the camera. For example, I have my front ring set to control aperture. As a result, the rear wheel defaults to control shutter speed. And, as if that weren't enough manipulation, an easy-to-access programmable shortcut button allows me to change the ISO with the flick of a finger. In the days of film, a photographer would be stuck with whatever speed was in the camera. When I used to carry my Olympus OM-2 everywhere, I usually went with 200-speed film. It provided some flexibility in low-light situations, while also allowing for high-quality outdoor shots when lighting was optimal.
With the Canon S90, I now have the portability of a point-and-shoot with a full array of easy-to-manipulate manual controls previously found only in SLRs. While it's going to take me a little while to learn how to wield all of the camera's features to their potential, I'm looking forward to becoming acquainted with this new camera that's willing to let me be an equal participant in composition process.
If you're seriously considering purchasing this camera, I can't recommend enough Richard Franiec's S90 Grip. Aside from making the camera more sturdy to hold, it's also made it more enjoyable to use, as it really feels right on your fingers. Plus, it looks cool, and appears to have been part of the camera's original construction. The image of the S90 above features the grip. The first link in this post takes you to Canon's website where you can see the camera without the grip.
S90 image from lensmateonline.com, accessed 1/1/10