Personally I prefer keywords. For a start, they are portable. They can be written to the photo of you export it. They are a Standard, and there is a place in the IPTC metadata for keywords. Faces are not standard. No other app will understand iPhoto's Faces data and vice versa. There is no place in the metadata for Faces information. Faces recognised on iPhoto will not be respected by Picasa online for instance, and so on. In fact, even Apple's own Aperture has n o understanding of them.
All of which is not to say that Faces are useless. They give you
another way to search and categorise your Library and if you enjoy that, by all means use it. It's quite possible that if I had not
a; Previously created and applied a keywording system to my 20k image library
b: a long standing interest in keeping my data portable
that I would have adopted Faces.
Of course, you can use the Faces results to keyword pics with a persons name if you want to. Not so interesting for me I confess.
FWIW: My keywording system:
I keyword on a
Who
What
Where basis (The When is in the photos's Exif metadata). I also rate the pics on a 1 - 5 star basis.
Using this system I can find pretty much find any pic in my 20k library in a couple of seconds.
So, for example, I have a batch of pics titled 'Seattle 08' and a typical keywording might include: John, Anne, Landscape, mountain, trees, snow. With a rating included it's so very easy to find the best pics we took at Mount Rainier.
File -> New Smart Album
set it to 'All"
title contains Seattle
keyword is mountain
keyword is snow
rating is 5 stars
Or, want a chronological album of John from birth to today?
New Smart Album
Keyword is John
Set the View options to Sort By Date Ascending
Want only the best pics?
add Rating is greater than 4 stars
The best thing about this system is that it's dynamic. If I add 50 more pics of John to the Library tomorrow, as I keyword and rate them they are added to the Smart Album.
Keywording takes time to set up, there's no doubt about it. I use
Keyword Manager as it's much more powerful than the inbuilt system, and adds the possibility of nested keywords. So, for instance, if I add John to a photo it also adds 'Family'. Now I can add many keywords to a pic quickly.
In the end, organisation is about finding the pics. The point is to make locating that pic or batch of pics findable fast. This system works for me.
Note that in that system you could easily replace the keyword for the person with their Face in a Smart Album.
Regards
TD