I have started using at the same time Twitter and Jaiku. Whereas the benefit of telling the world what are you doing/thinking in 140 characters or less may be dubious for some, I find it quite addictive.
Since both services are very similar, there is some healthy competition between them. Twitter has going for it the fact that it's very heavily used in the States, and, crucially, has Facebook integration. Jaiku, on the other hand, has a more European user base, the interface is in my opinion more appealing and, interestingly, has the option to add other RSS feeds, so you can see not only my Jaikus but also my blog posts, public Flickr pictures, the music I've been listening to (thanks to Last.fm) and even my Twitter posts at the same place.
Both also have Widsets available to be used in my N93 (review upcoming), so I can update my status on the fly.