If you're a techie who needs to actually get work done on hard technical problems, it's a total disaster, time- and distraction-wise.
One analogy would be the tech pundit (forget whom it was) who said reading the NYT at breakfast would send him into a mental maelstrom for the rest of the day, on overload. Twitter is like 100 micro-NYTs. ;-)
At least that's my (admittedly short stabs of, now and then) experience.
[Edit:] I suppose if you had a reasonably-sized close group of associates who only twittered a few times a day, you could keep it under control as a 'water cooler' experience.
In the end, it's what's valuable to you. If you can't (or don't want to) find a group of people to follow that you find interesting, helpful, or whatever, that's fine. Just because you (or sharpn, or my wife) don't find the service valuable doesn't mean that there's no value in it for anyone, though.