Overall conclusion
There's been something of a 'serious compact camera' renaissance in the last couple of years, led by Panasonic with the LX series and Canon with the G9/10/11. That said, the choices for the enthusiast wanting full photographic control in a pocketable package are still very limited (even the LX3 is big for a compact), meaning the S90 was given a warm welcome when first announced last fall.
On paper it's a compelling package, with comprehensive photographic features, a fast, useful zoom and as much in the way of external control as you could possibly fit onto such a small body. For the SLR user wanting something that offered the 'carry anywhere' convenience - but not the compromise - of an ultra compact DSC the S90 promises to be the perfect second camera.
And for the most part it delivers on that promise; the image quality is surprisingly good, the user interface well thought out and the design truly pocket-friendly. The handling takes some getting used to, which I guess is inevitable given how much Canon has tried to cram into a camera the size of an Ixus/Elph. But once you're familiar with the controls and the various operational foibles it offers a surprisingly fast and fluid shooting experience for the enthusiast photographer with a desire for total photographic control.