Well the voice recognition in Vista is pretty good in my experience, but it depends greatly on your voice. I have a pretty strong voice so it works well for me. On the other hand I've watched a quadriplegic woman with a very soft voice try to use it and it just failed miserably, which is sad because she could really use it.

I think you will find that when it's listening for specific commands, it's almost 100% accurate unless you have two very similar commands, something like "Send" and "Sand". The dictation is hit and miss still on mine. I think the more you train it outside of my program, the better it will get.

As far as how an accent affects it, well your profile says you're from the UK and I see an option in the voice recognition settings for UK English instead of US English. If you have an accent because English is a second language, I'm not sure how well it would handle that.