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Almost perfect client, definitely recommended..
This twitter client is the only AIR-based client I tried using that works with the new API. It has a pretty, minimal and clean interface, easy configuration, filters, drag-and-drop pictures uploading and other nice features. It has also a few different themes. The only thing that's missing is auto-complete for mentions, which I'm waiting to be implemented. Other than that, it's a really valid alternative to any Twitter client out there, and best thing of all, it's free.
Pros: It's fluid, simple and works perfectly
Total control on fonts and notifications
Cons: Missing an auto-complete feature for mentions