I've been using Picasa lately as my primary photo viewing software. I like it for several reasons

• It views .cr2 raw files from my Canon EOS Digital Rebel XT (350D) directly, without the need for any extra conversion steps. Since the 350D doesn't take RAW+low quality JPEG shots like the 20D can, this is a must-have feature for me.
• It's fast, even though I'm using the Linux version, which is just the Windows version running in Wine.
• The UI transitions have all sorts of fancy eye-candy animations.
• It's designed not to touch the original files, which is great when the original files are lossy-compressed jpegs and any change would lose image data.

However, it has some drawbacks:

• The white balancing eyedropper tool doesn't seem to be as good as a dedicated raw file converter. I have a very dark shot including a pool table which includes an orange "5" ball in the foreground and an orange "13" in the background. When I white-balanced with ufraw's eyedropper tool, both balls came out roughly orange looking. With picasa's eyedropper tool, the "5" came out bright yellow.
• Sometimes, a file gets copied or moved into an Originals directory for no reason I can discern. I think there must just be something I don't understand yet. Most edits/exports do not cause this to happen, but every so often, it does happen.
  
• The cr2 metadata (aperture, shutter speed, etc) doesn't display, like the jpeg exif metadata does.
• The Linux integration isn't perfect. It would be nice to be able to "Open With Gimp" directly from viewing a picture.