a lot of what's on twitter is alleged to be fairly inane - ululations in the dark, and so on. Maybe it is, maybe it isn't, maybe just because I find something boring and uninteresting doesn't mean it isn't important to that person.
However, it's fairly clear that a lot of people have similar doubts, and for that reason we get twibes and lists to let people find communities of interest, and that's a very interesting phenomenon, that the need of groups of like minded people to contact and share with each other is sufficiently powerful that it supports self organising add on technologies.
The other thing is that make use of things like twibes is a fairly deliberate action, its declaring yourself a member of a community that wishes to exchange links and news about particular topics.
The other question is, will these self organising groups end up like either usenet - a minority playground for trolls and sad anoraks - or like some of the specialist mailing lists, little inward looking communities of self selection - or will it turn into something generally useful and self sustaining?
Twitter is clearly useful - as a way of getting instant information and sharing links it's invaluable.
One of the nicer uses I've seen of it is by the The computing service at york where they use it to get service status updates out.
This is particularly nice as (a) it means that even if everything is down you can get a service update as it's independent of all local servers, and (b) it provides an rss feed making it easy to repurpose into other things - such as displaying on a faculty web page.
As usch it's agenuinely useful use of twitter. As to twitter itself, it has also gone through massive growth in the last eighteen months and is continually changing through the development of add on services. So far the signal to noise ratio seems to be acceptable ...