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.
Brizzly's rerelease (not an update; Birdfeed is no longer in the store) of Birdfeed changes the name, tweaks the UI, and slashes the price down to zero. The interface isn't as dazzling as it was before, and Birdfeed's trademark lack of a menu bar has given way to a standard row of icons. Brizzly actually adds a few new features, including a trending topics-esque News tab, for explaining what's going on in your feeds, and the same lovely pull-down feed refreshing as the other best iPhone Twitter app, Tweetie.
Even its apparent shortfallings aren't so bad: Yes, you have to sign up for a Brizzly account in order to use the app, but one you have, it's completely transparent. It's like Meebo in that sense. And no, the app doesn't have push notifications of its own, like Echofon does, but premium Twitter apps have long offloaded that responsibility to dedicated push apps like Boxcar. (Which is great beyond Twitter, by the way.)
In short, if you need a Twitter app but don't want to pay, Brizzly's the one. [iTunes]