Recently David F. Gallagher wrote in the NY times about Google voice. David asked people to leave him messages and he posted the automatic transcriptions. While performing good in many cases, the results are far from accurate and in many cases one cannot understand the gist of the message.
See information at pushing the limits of googles speech recognition and messing with googles speech recognition part 2.
To some extent everybody knows it is experimental and do not really trust it. Is it useful? Probably just for fun and for people who do not want any human to be involved in their private message transcription like SpinVox, Nuance or other such vendors.