In the past I tried tagging my photos with tags corresponding to the location, major things and people in the photo. This was mostly a waste of time. Folder names were already sufficient for finding photos by location and I wasn't interested in finding particular types of things (eg. lakes) in my photos. The only useful part was tags corresponding to people.
The last tagging was a long time ago, and so I had a lot of untagged photos. Recently, I felt like adding tags corresponding to people, but I assumed that it would be very tedious. Then I learned that Picasa has facial recognition and I decided to give that a try.
Starting the process was surprisingly easy. When I added a folder to Picasa, it first scanned the folder in the usual way and generated thumbnails. Then it started scanning the photo for faces. It quickly started filling a list of faces, and I just had to start identifying people. After a bit of manual identifying, Picasa was able to automatically identify some people.
There were certainly some issues and limitations. More than a third of the time, Picasa found a face but failed to identify the person. Occasionally, a person was misidentified (or maybe it was a sign that I look like some relatives). Then there were all the strangers in the background, whom I had to remove from the list. The most interesting issue was occasional identification of faces in artwork (eg. Mt. Rushmore, paintings and T-shirts) and even patterns of reeds or clouds. Fortunately, dealing with all these things was fun.
When the scan finished I wanted to see how many faces were missed. At first I was somewhat disappointed. Picasa easily dealt with sunglasses, but tuques or harsh shadow boundaries often caused faces to be ignored. Picasa also seemed sensitive to face orientation. It did well when a face was vertical and viewed from the front or side, but other angles were a problem. Manually adding people was a bit more trouble than it should have been. I had to go to the photo page, draw a box around the face and select the name. I wish I could have added tags from the thumbnails page without defining the person's location. After doing all this, I felt that Picasa automatically found the faces in the majority of the best photos. Even the automatic face detection wasn't anywhere near perfect, it was good enough and I was satisfied.
Picasa's automatic face detection also provided an unexpected benefit: the way it cropped faces was sometimes very aesthetically pleasing. It actually improved some photos, and it provided many images which could be used online.
Now I'm wondering whether to continue using Picasa or extract the face information and uninstall Picasa. I'm happy with the way I organize things into folders, and generally not interested in programs for managing libraries of files. I still have to see a bit more about what Picasa offers.