Considering that this suite of five social websites has been out there for 3 years already, it has taken me a long time to stumble across them. I signed up two weeks ago, which hasn't been long enough yet to use every feature on those websites but I do at least have the gist of what they're about.
43 Things is, IMHO, the only really useful service of the bunch. It is a social to-do list. You can start your own goals or join in on goals that others have started. You can write journal entries and attach pictures to each goal detailing your progress. You can give (and receive) "cheers", which are tokens of support. Once you've completed a goal, you can rate it as worth doing or not worth doing, optionally stating your reasons why. One thing lacking in 43 Things is there is no way to make a goal private. Since you may not want all your goals to be known to the public, that limits its effectiveness as a to-do list.
43 Places is like 43 Things but for places and events. You can post towns, venues, stores, restaurants, or events to which you're planning to go or to which you've been. You can rate those places as worth visiting or not worth visiting and you can add pictures. In practice though, I've found it to be more work than I thought to add places to my list. The search feature didn't always return the results I wanted. Oftentimes too, the town or county would be missing and I'd have to add those to the hierarchy before I could add the place.
43 People is the same idea applied to building a list of people you want to or have met. I wasn't interested enough in this service to take a further look.
All Consuming is a website where you can track stuff that you intend to consume, are consuming, or have consumed, and rate them as worth or not worth consuming. There is a broad range of stuff that this website considers consumable. You can consume books, movies, music, food, or the catch-all "other". Within the "other" category, you can consume everyday items like socks or intangibles like irony, Flickr, or All Consuming itself. (in a self-referential manner) All Consuming seems mostly frivolous, except for the books/music/movie section where you can build your online catalog and see who else has owned and/or commented on the same titles.
List of Bests is where you can make lists of books, music, movies, places, people, and other items (all tied to entries on the other websites in the suite), and keep track of which of those items you've consumed, visited, or met. I haven't been using this website much either.
To summarize, the social aspects of these websites are fun, although there is a danger that they'll be a big Web 2.0 time sink. However, I do think that there is something to 43 Things if putting your goals out in public helps you get motivated.