Extremely simple and easy to use, Vlingo's iPhone application blows away its rivals--so far. The free service lets you speak your search, contact names, and Facebook and Twitter updates into the phone, featuring great service support from the get-go. It also includes basics like speech-to-text searches of Google maps, contact dialing, and queries of multiple search providers.
Like most iPhone applications, Vlingo could use a built-in browser. It never crashed on us, and fails with grace when you've reached a data blind spot.