I was mostly using the web interface of twitter till last week.
I was searching for a good twitter windows-client and after a while, I have found that twhirl has a very good echo everywhere. It is also the first Adobe AIR application that I am using.
And, just after some successful days of usage, I have read that Seesmic is acquiring twhirl! Cool. More information about the acquisition by Loic.
My twitter account is didierbeck.