It's not that Brizzly's perfect, or that it does justice to its source material (the unassailably pretty, wonderfully lean Birdfeed)—it's that it comes close enough, and it's free.
First, a quick lesson in the history of iPhone Twitter apps! Once, there was an app called Birdfeed. It was clever, fast, and visually distinctive. In fact, it was (up until just now) quite possibly one of the best Twitter apps available. It was also expensive, at $5. The developer, who was tired or something, sold the app to a little startup called Brizzly, which aggregates Twitter and Facebook feeds into a single interface (it's actually kinda neat, as an online service.) And so here we are.