I have only started to use PhotoLinker to tag my photos. It uses industry standard IPTC tags to set metadata for photos (ie, what Adobe does). I am using it to both georeference photos, as well as to maintain metadata.
For the georeferencing function, the user interface is clean and very easy to figure out. The issues of setting the camera's time zone, and any errors in the clock, are well thought out. The program uses Bing maps for georeferencing, which are a little slow and less useful than, say, Google Earth (which HoudahGeo uses).
The details of metadata for photos are very complicated, and, while I have read about them extensively, While I do not purport to be an expert on xpm metadata, when I communicated with the author about some of his decisions, I found him open to suggestions and quite well informed.
The program is being actively maintained, and a Lion (beta) version came out before the operating system was available. I am still using Snow Leopard, so I can't speak to Lion compatibility, but the message boards seem to indicate it is working well.
While it is expensive for what it offers, it is very well designed. If you are just going to georeference images, there are cheaper programs. But if you want georeferencing, plus a clean way to maintain metadata for large numbers of files, this is your program.