I'm using SAPI speech recognition in my home automation setup (an Epia system). I've found that the more you train, the better it gets. Mine's pretty accurate for the stuff I use it for (stuff like, "Computer, turn on the porch light." or "Computer, what's the temperature outside?", etc). I'm sure it would be similar for a car environment. A good mike helps too.
However, it trains to your voice only, so if you have other users using the system, it makes things more complicated. A workaround for this is to do the training with the other person together, e.g. each person alternates reading the sentences.
(In purchasing stage for my carputer...almost ready to start install).