I've been back and forth about Twitter (my page is here). At first it was neat but useless to me. Then I had a couple of friends in there that I was interested in keeping in touch with. Then I lost interest in it, then I went back. Sometimes I can't believe I'm bothering with it, other times I wish I had more friends. Kind of like real life.
Twitter's certainly a decent idea and I believe it's on to the next evolution in IM. But if you can't convince all your pals to get a Twitter account and deal with the hassle of updating it, then you're not going to get much out of it. Which leaves only hardcore net nerds talking to themselves... to each other.
Tumblr's been fun and I'm still using it (my page is here). I don't know if anyone's reading my tumblelog but I feel like I'm creating a little meme repository that I'll be interested in looking back on in the future. So it has some inherent value to me as a user regardless of whether my friends sign up.
It's desperately lacking tagging. I like its simplicity but the addition of tagging wouldn't overly complicate it and would add so much to the service. Different ways to view my data, ways to view site-wide tumblr entries under tags, etc.
While both sites are nice, ultimately I'm exhausted with new services and would like to combine them. I was originally thinking that Twitter and Tumblr would go great as a combined service, which touches on my semi-obsession with a temple of ego or Mugshot style service.
Steven Garrity pointed out Jaiku to me last night, which (I'm assuming) is a European Twitter clone. But instead of simply being a one-off status posting service, it also merges all your feeds (del.icio.us, flickr, picasaweb, blog, lots more) into one timeline that unifies your online identity.