It worked if you used a headset/mic and didn't have a cold; otherwise background noise issues (ie if some noise was present when you created the login, it had better be there when you tried to login again. Similarly, new noise when logging in could cause login to fail) made it fairly useless. Simply moving the computer to a different part of the room or building could cause it to fail. No music or radio allowed, either.
It wasn't voice recognition so much as an audio pattern recognition; you had to repeat the phrase exactly (same inflection, speed of delivery, etc). True voice recognition "knows it's you" no matter what you say or how you say it.
I tried it for a while, it certainly was "cool" but I'm pretty sure around 100% of users came to the conclusion that it wasn't a better way than typing a password. I don't think it's a suprise that Apple didn't spend much time coding for an OSX feature with no earthly use.