Hi Gideon,
I've been on Goodreads since the beginning of the year. I really enjoy the site. Of course, as an author, it supplies some promotional visibility for me because people at least see me, but I mostly use the site as a reader.
I like the "to-read" list because I can stick titles here that I am interested in. Because so many books catch my eye, it's a nice list. This way instead of forgetting what I think is interesting, I actually have a place to refer to when I am looking for something to read.
I also peruse the ratings and reviews of the other readers that are my friends. I usually form a reading friendship if I see a review that interests me or someone tends to read the type of books I like. This way I get exposed to new titles on a regular basis and can use a friend's rating to gauge potential quality.
This is my goodreads URL:
http://www.goodreads.com/profile/Tracy_Falbe