I have used Digikam because I thought it was 4 years newer but it was a complete and utter disaster. Huge problems (slow) when using it with a lot of pictures, face detection all over the place and a shitty interface to tag people.
Picasa on the other hand has no problems with huge libraries and the tagging interface is actually fun to use.
Downside to Picasa is that the initial indexing is very slow, as it only uses 1 core.
Also (both Digikam and Picasa do this) software needs to write metadata into the JPG file. No sidecar allowed.