Over at the Postbox blog, they're teasing the upcoming version 2.5 with a post about a New Addressing Widget. There's only a screenshot, but it looks like a nice usability improvement - it looks more Mac-like. To and CC are in separate fields, so it's easier to add a CC without using the mouse now, and they use the token field widget (the little blue lozenge introduced in Apple Mail in 2005) to make it easier to drag around whole addresses.
They do say they're working on some re-styling and finishing touches - so I'll reserve judgement until I use it. For now, it's still catch-up.