One major use I have for it is determining all the photos where one (but especially two or more people) appear. This is very useful for prints, photo books, or just tracking down those photos you forgot you had.
The first option I've found to help migrate Picasa Face Tags into images directly is with AvPicFaceXmpTagger. The summary below (from its wiki pages) gives an idea of what it does.Click on the title for the creators website/download.
The program seems to work with DNG files as well, from the tests I've done on it. As always, back-up your data and catalogue before trying "new" things like this.
The next tool I'm looking into for this process is Jeffery Friedl's "Picasa Face-Recognition Import” Lightroom Plugin which seems to do much the same thing, but with a bit different final results.
This program was written because of the new Google Picasa 3.5 face recognition feature. Due to the fact that Picasa 3.5 doesn't store the face recognition data inside JPEG pictures, this program fetches that data and stores it as XMP-MP meta tags inside the JPEG pictures.
The Picasa 3.5 face recognition feature stores the face data inside the Picasa database. As a consequence this data is lost when you give away your pics and this data cannot be shared with other users or with other Picasa instances running on different computers. Luckily Picasa stores the face recognition data (persons & faces) in quite simple text files so reading that data is not a big clue.
There is already a method available how to store face recognition data inside JPEG pictures. This method uses XMP tag system created by Adobe with the addition of Microsoft XMP namespace Microsoft Photo Region Schema and is used by Windows Live Photo Gallery.
Right at the start this program reads the Picasa text file holding the persons (contacts). Each person has assigned an unique person ID. For every file of for all files in a directory which is added to the list of files or directories the picasa.ini file is searched for face definition. If face definitions are found for a file, they are added as XMP tags. Optionally, the person names are also added as IPTC Keyword tags.
The actual tagging of the JPEG files is done using ExifTool - a great and widely accepted/trusted tagging program which is known for its good makernotes handling (to leave them intact).