It's notable that Microsoft is going after iPhone mind-share after Steve Ballmer famously dissed the iPhone more than 2 years ago. He's had to sit back and watch the iPhone eclipse Windows Mobile phones in the marketplace, which can't be pleasant, but business is business, and Microsoft is pushing the Bing search suite of tools hard.
After spending some time with it last night and this morning, I can say that it works pretty well. I think on-screen it is more attractive than the Google Mobile app offering [iTunes link] and the voice control is about as accurate.
On the Bing home screen there are direct links to images, movies, maps, businesses, news and directions. It inverts the Google method, where you do a search, then select, for example, images. I think I prefer the Bing hierarchy as being more logical.
The voice search isn't perfect. It especially had trouble with addresses. You can use Bing, as you can Google Maps, as a low end navigation app, but of course no voice directions as yet.
Some of the icons, like back and forward seem a bit too close together, but the icons are serviceable. Like Google, you can search for a business, then click to dial it directly, get directions, or read reviews. Business categories are well thought out, and easy to navigate.
You can access an iPhone friendly version of Bing from Safari on the iPhone, but of course there is no voice input and it is more cumbersome to use, so I think having an app makes sense.
For free, there is no reason not to download Bing and see if you like it. I'm hoping Microsoft will aggressively update the app, and continue to add features. I think if you grab it, you'll use it. Bing requires an iPhone and iPod touch with iPhone OS 3.0 or later.