Ignoring the "hidden" nature of keywords, there's some pretty slick things about it. I like that the captions and the tags get written to the IPTC headers. This is nice in that it means that if I migrate away from Picasa, any other IPTC aware app will be able to read the tags and captions. It's also pretty nice in that when I upload photos to Picasa Web Albums (their online gallery space), the tags and captions from Picasa get sent as well. Apparently Flickr parses IPTC data as well, so if I ever do switch to using Flickr (doubtful due to the limitations on their free account), the tags and captions would go with the images.
In terms of more slick, seamless integration, I'm pretty impressed with my SD800. It's got an orientation sensor in it, that writes to the EXIF metadata. The image viewer I use reads these tags and rotates the images accordingly, as does Picasa (I'm not sure if the files are actually saved rotated?). When I upload the images, they're automatically rotated as well online. Kind of nice not to have to go through all my image thumbnails and rotate accordingly . . .