Lots of new people are trying out Jaiku after having used Twitter. Many good things will come of this.
Joi Ito puts the point well (excerpted below)
To me, Jaiku, which existed before Twitter, is a bunch of Helsinki mobile jocks getting into the Web 2.0 of it all whereas Twitter is the Web 2.0 crowd "getting" co-presence...Twitter was funny for me because it was like the whole "laptop crowd" getting the "aha" that Europe and Asia had with SMS awhile back - the idea that the Internet isn't about "cyberspace" that turns on when you open your laptop, but that the Internet was something that you could carry around with you and that could ping you when it needed you.
The media - and the blogosphere is no different - loves a good conflict. I think it's a mistake to position Twitter as a copy of Jaiku, or the other way around. As Joi says, the two are coming into the same field from two different directions:
Jaiku comes from a "presence" background allowing bluetooth proximity, phone idle time, ringer mode and other things to trigger state changes - the messaging came later. Twitter, on the other hand, is primarily messaging, which as we all know, is just a flexible and manual vector for presence information.
To this, let me add a personal note on why I use and will continue to use Jaiku. It has to do with the user experience and particularly social experience of Jaiku. Note that I don't have a Twitter account. I looked into getting one but I don't have time to update in two places. And those that know me, know I like it slow sometimes. Twitter is faster and louder than I like my mobile social life to be.
For me Jaiku is about:
1. Silent sociality - checking up on what my friends are up to when convenient, and posting my own state knowing that I won't be disturbing others (unless they have explicitly asked to be alerted).
2. Small-group sociality - Jaiku is not about celebrity. I'm interested in sharing state with a small group I'm nearly always in contact with, what Mimi Ito has called full-time intimate community.
3. Mobile sociality - Jaiku was designed with the mobile "living phonebook" interface in mind. SMS alerts crowding the inbox of one of the few working personal and functional communication channels is not my idea of improving communication. I use the SMS-in posting to Jaiku when I'm using my Nokia 8800 and with my N70 I use the Jaiku phonebook.
4. Background sociality - Jaiku allows me to integrate other online identities and feeds (including delicious, flickr and any RSS) into my single jaiku presence feed. This is done in a way that doesn't confuse these background posts with my explicit state messages.
These four features are the reason I use Jaiku.
This said, I think both Twitter and Jaiku can and will learn alot from each other. What Jaiku can learn from Twitter is the simplicity of use and clarity of design. Twitter, from what I know of it as a lurker, would do well to watch Jaiku in order to understand mobility better.
Disclamer: I am an advisor to Jaiku together with Joi Ito.