Have you ever had a left a message for a friend with google voice trying enunciate clearly, knowing it was being transcribed? Ever gotten a voice mail from an automated system? Those transcriptions are usually pretty close to perfect. Having used 2.1 (at least on the Nexus One) a few times, I can tell you if you make a reasonable effort to speak clearly, it does a really good job. Google Voice fails b/c people aren't trying to speak for a transcription (not that they should need to, but I'm defending the engine, not the service).
Well, my voicemail message doesn't currently have a "your message will be transcribed, please speak slowly and with as little accent as possible" disclaimer in it. I suppose I could add it but that seems a bit akward.
If nothing else, it can be VERY entertaining to read the transcriptions. I don't think I've had one yet (out of hundreds) that was entirely accurate unless it was a simple "call me back".
I have used other transcription tools on the PC and I can say that Google Voice is less accurate than PC transcription tools were in their infancy more than a decade ago. It's also the only one I've used that I didn't have to pay for however. I still use it and like it but I wont deny that it has a long way to go in terms of accuracy.