I've just joined Jaiku after getting an invite from Cormac Moylan – thanks Cormac.
Jaiku is similar to Twitter apparently, though I've avoided Twitter thus far. I guess the attraction of things like Twitter and Jaiku is something to do with the very transient nature of adding content.
You drop a quick note in as to what you're up to, something you've just seen, a quick link to something interesting, and someone might pick it up, they might not.
It certainly seems as though there are a lot of options for getting that info on to Jaiku. I can use gtalk (or most other IM) to send a message, or type something into the web interface, or I can use their S60 mobile app, or, if I really wanted to, I can send a text to a mobile number in Finland (thank you, but I think I'll avoid that one for the time being).
Other than that, in your account settings you can add in the addresses of any feeds you have, from photos to blogs, to Last FM, and it automatically updates your Jaiku page with this content.
I could see it being more useful if there were loads of people I knew on it, but for the time being I'll be content with linking in with a certain Irish web crowd and keeping in touch with what's going on there.
As noted, I haven't checked out Twitter, but according to a few of the Irish heads on Jaiku, the functionality and display of information on Jaiku is quite superior to that of Twitter. I won't actually state that until I check it out myself…