Google's voice search capability is available on the iPhone. Based on this review, it appears to work well. "Works well" could mean a number of things, though. It could mean "the recognizer works well" or "it finds a lot of interesting stuff" or "it helps me do things I really need to do." The last criterion is really the important one. It's the thing that will draw people to use the service frequently, and perhaps generate revenue.
Readers' reactions to the app is really interesting. Most of the reactions are pretty positive, but a few people registered complaints. If you work with speech reco technology you know that the underlying language model is based on North American English. You realize that it doesn't work as well with people who are sick with colds. But the readers didn't cut the app any slack for those things, and that's important to remember for people who are delivering speech apps. Users' expectations are already pretty high for speech reco, and this will make it even higher.