The Google iPhone app has leaped back to number one in the free section of the iTunes apps store, in large part based on the widespread coverage of its new voice search capability. I don't have an iPhone. I have an iPod Touch so I'm unable to use voice search because the device doesn't have an input mic. I had to go to an Apple store, download the app onto one of their demo iPhones and try it in the Apple store.

I was somewhat disappointed with how it performed in that relatively noisy environment. Only about 50% of my searches -- and I tried lots of different kinds of queries (local, commercial, informational) -- were correctly served. 

This is the beginning and so it will get much better over time. But from my limited experience it was not the breakthrough in terms of accuracy that I had hoped. I did however really like the ability to simply move the phone to my ear, wait for the beep and speak. 

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Here's a formal statement that Google offered regarding why the voice search app has trouble with British accents:

The acoustic model for Voice Search was trained, in part, by using data from GOOG-411 which has only launched broadly in the US. Since the acoustic model was trained using mostly American accents, the tool currently works best when receiving queries with American accents. While you can still download the Google Mobile App and turn on the Voice Search here, we've turned off the voice functionality by default when the app is downloaded from anywhere outside of the US. We don't have any specific launches to announce at this time, but we think this is exciting new technology and the speech recognition and understanding will only get better for other accents and jargon as we keep working on it