I've been using the S90 for a couple of weeks now, so I thought I'd post an update.
First of all, I've been getting majorly annoyed at the huge delay between taking a photo and being able to adjust the settings for the next pic. Usually the workflow is like this: take photo, review, adjust settings, take next photo. The time between viewing the image on screen and being able to use the dials again is just too long. I want to be able to adjust the exposure compensation immediately when seeing the last photo on screen. I pray to Canon that they can make the camera more responsive in the next firmware update.
Speaking of exposure compensation, I've changed the lens ring's default function to adjust the exposure. In P and Av mode I adjust the exposure compensation with the lens ring, and in Av mode I adjust the aperture with the back ring. This way is really intuitive for me to use, and I like it a lot.
Last thing I like: adjusting the white balance with both control rings is absolutely brilliant. It's a feature I never felt I'd need, but now I can't go without it. I wish my 50D was as easy to adjust as the S90. It's so intuitive.
Update: I shared some photos of my cycling trip today here at Picasa. All photos taken straight from camera without any editing.