I think if you're a social "hub" yourself, such as an Arrington or a Gruber, it's ideal.
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.
