My ten cents:

If your whole life is JPEG pictures, then Picasa is pretty damn good. You can write arbitrary text into the captions and do full-text search on them. Picasa doesn't play nicely with network shares, but it does rescan JPEGs insanely fast. The guy who wrote Picasa was a serious performance nut, so everything it does is just stupid fast.

If you want a workflow involving raw images, sadly, you should forget Picasa and use Adobe Lightroom. Lightroom 3, in particular, finally has decent noise reduction and lens distortion correction built-in. You can also get plugins that make it easy to export to Flickr or Picasaweb or whatever else. Lightroom has an insane amount of control over metadata. You can apply arbitrary text tags to images and later search for them. You can also tag with stars (0-5), flags (thumbs up or down), colors (five or six of them), and title/caption.

Keep in mind that Lightroom requires a much faster machine to make up for its inexplicably poor performance.