Picasa Web just launched their "NameTag" feature which automatically detects faces and categorizes them in the entire album. The user interface to browse through the detected faces and for tagging is very intuitive. The face detection and categorization engine is very accurate.
This functionality along with GeoTagging, which Picasa already provides, fulfils the typical user requirements for photo sharing and management.
User Interface for tagging: The interface is very neat and easy to use.
1) A carousel with the lead face from a face category allows easy navigation to a particular person. For that face, the user can then select/deselect particular pictures and assign a "name".
2) The selection of name has 3 different modes -
a) selection from a suggested list (picasa's guess of who the person is based on previously assigned names),
b) selection from a list of contacts existing in either GMail or in Picasa Contacts,
c) and creation of a new contact.
Selection from Picasa's guess - 
When it is difficult to identify the person without seeing the entire photo, Picasa allows viewing the picture in an AJAX popup.
3) Undo and "Merge" people functionality is very useful when correcting tags
Face Detection: BRILLIANT!
1) They are able to detect faces from even low quality and dark pictures
2) The categorization is able to group together the faces over a long period of time, irrespective of the person's age in the photo. The photos below span a period from 1980-1990, and still the algorithm was able to match it to the right person (my dad) who has changed a lot since.
3) The auto suggest for names (based on previously tagged faces) is correct 80-90% of the times
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| With Makeup | Age | Obstruction and Head Gear |
4) The auto-suggest of names usually suggests names that are somehow genetically related - which is very interesting - the algorithm finds faces to be very similar within a family
In the following picture, the face shown is a sibling of the name suggested as the closest guess by Picasa, considering this picture was taken about 20 years ago, the matching is amazing.
Even when there are errors in the auto-suggest, most of the times - one of the suggested names is the right one. A lot of times, the faces (even a whole category) are suggested to be a sibling.
In the set below, the 3 faces belong to "Nishith" (second suggestion), who is a sibling of "Anshuman" (the top suggestion).
5) The "Find More" feature works well when one wants to tag for a specific few people in the entire album