Speech recognition on Windows Vista is exceedingly good. My initial attempts with using it were successful but I did not attach much importance to it since I did not see myself using it in day to day life.
I'm not really sure what has changed, but I've been using it all of today for pretty much everything I do on the computer, which includes typing emails, chatting and suchlike.
Consumer acceptance with regards to speech recognition reached a plateau a few years ago when it seemed like it was universally prevalent on most operating systems, but it never took off because accent training was impossibly hard. Vista seems to have cracked the barrier of intuitiveness - -right out of the box it seems to comprehend any normal accent, and seems to interpret words based on the context. This is awfully hard to do and terribly useful if implemented well, as Microsoft has. For one thing, it distinguishes between speech that is meant to be interpreted as text and system commands. The system commands are not rigid, which means you can pretty much use a logical command and expect the system to comprehend. Furthermore, there is a comprehensive set of navigation aids that allow you to use your voice to achieve a remarkable dexterity in navigating the computer. For example, in the above video you will see a demonstration of using the speech recognition to draw a line using MS paint.
Personally speaking, this is the first time for me that speech recognition has extended beyond an interesting plaything to being a useful tool. I can see myself getting by with speech recognition for most chores where I would have needed to use a keyboard. This is really helpful since I have been using a keyboard for a very long time now and I fear carpal tunnel syndrome and RSI as an occupational hazard.
It works well for now with Microsoft products such as Internet explorer and badly with a browser like firefox. This is to be expected for a number of reasons, but I am not going to complain about having to type my letters in notepad and copy them into gmail. This is my first day and the experience is already quite good, I can only imagine that it will improve with further training.
Mac OS X (leopard) has speech recognition for system commands as well, but I can tell you that it isn't half as good as vista. Which is a pity, because I have come to depend upon the Mac for most of my work. Luckily I also have a Vista desktop at work which I hope to capitalize upon. Give it a try! Soon, you will also sound like a robot, just like me :-)
P.S. This post was not composed using a keyboard.