I'm using S90 for few days. Images are a little soft but nothing really bad. It's quite easy to use.
Overall exposure is ok, but it does blow the highlights easily when sky is in the picture (in anything that's not full manual). You can control it with exposure compensation or by exposing for the sky.
In auto has good WB, and if you shoot in jpeg the results are good.
In raw though, there is a huge amount of barrel distortion. Also a lot of purple fringing. Processing is slow if you want to adjust each picture as you have to use the software supplied by Canon. However it does a good job, getting rid of the barrel distortion and most of the chromatic aberration. Overall results are quite good for a camera that is pocketable and has manual controls.
There are some usage issues, such as the too easy to move wheel from the back, or the flash that goes up exactly where your left index is. Nothing too bad though.
So far I took pictures only indoors and outside in a forest, at sunset, pretty low light.