The iPhone app is purely functional: you can review available cars, locations, make reservations, and actually make the car horn honk when you're nearby looking for it. The interface is simple and utilizes the bands of green that appear on Zipcar's web site, so it's nothing special to look at. It's not funny, and there's nothing viral to send to a friend. But it's brilliant branding.
Doing things that matter to consumers is the great, undiscovered country for brands. The days of being witty or otherwise different with advertising copy or grand PR declarations are long gone. People don't believe it what businesses claim to be true, and there's an inverse relationship between the entertainment quotient of the inputs into social media, and the relevant actions that come out of it. Talking about a funny video just doesn't equate to effective marketing anymore. I'd argue that it never did.
So for every hundred dumb "advergames" that companies propagate into the cosmos, there's usually one app or online service that adds value to consumers' experiences. The Zipcar utility (which is what it is, really) is one of those exceptions.
It's interesting that consumer reactions to it have not been consistently good; as much as half of the numbers reporting the top rating give it only one star. Their biggest problems aren't about anything to do with creative marketing (the jokes weren't funny, the socialness wasn't social enough), but rather that the app doesn't do enough! The consistent complaint was that it should replace the zipcard members must still use to rent vehicles.
This is exactly where brands want to have conversations with their customers. Reality. Doing things. It's why the Zipcar app is brilliant branding.