I had meta invite me in b/c I was curious. I'm also bed-ridden and am in need of any sort of activity that relieves my boredom. Orkut seems like a cross between Tribe and Friendster, with it's main advantage over the two being that it's really fast. Unlike Friendster, which takes a minute to load someone's profile, Orkut lets you hop around as fast as you can click. It also adds in the notion of karma (trusty/cool/sexy), which seems like a reasonable idea, but then you notice that a bunch of geeky, balding men are rated very sexy, and you have to wonder. (there's no disincentive to giving all your friends max karma)
Another thing we noticed is that the "fan" feature is kinda questionable. If you're already friends with someone, then it doesn't add that much to say you're a fan. If you're not friends, then it kinda comes off as creepy. As an example, meta and I noticed that someone had added himself to her fans list. We thought that was weird, so as a test I started clicking on photos of women I thought were cute. Sure enough, the guy was listed as a fan on all their profiles as well.
Orkut seems to be aware of the potential for these creepy situations, as the guy's profile has now been deleted with the useful message: "Removed by evil all-powerful Orkut workers."