I took a course for EMT-Basic training about a year and a half ago and my interest in breakers and cutters peaked. I never got certified, but I enjoyed the learning experience and got to spend some time in a level 1 trauma center which really helped me get an idea of exactly how messed up a bad situation can be and altered my mind set under pressure.
During the time that I was taking the course, I learned about the Benchmade Houdini when a friend picked on up and decided to check it out. Tested both the cutter and the glass breaker on it, and honestly I think there's absolutely NO EXCUSE not to carry one or something similar in your vehicle AT ALL TIMES. Just like in any defensive situation, being under pressure in a vehicle emergency while trying to help can really jack your fine motor skills. Using a regular knife blade with a point while trying to get somebody out of their seatbelt can end badly whether you know your tools or not. I'm not saying to carry one in your pocket, but keeping something in your glove box is a BIG recommendation from me.
As for the function, the glass breaker worked maybe a little too well. It shattered every piece of glass I tested it out on, cutting my hand in multiple spots doing it the first time. If you get one, wrap a T Shirt around your hand or something to protect it from glass because forward momentum will take your hand through a window you're trying to break. It's also best to break a side window and to do so from a bottom corner if you need to get somebody out of a vehicle. It's a good idea to break by the door lock not only to unlock the car door if needed, but because there's a chance the door itself will stop your hand before it goes too far through the window.
The seatbelt cutter is a simple item. If you ever need to use one, it's going to slice right through easily. Seatbelts are actually quite easy to cut.
EDIT: By the way, this isn't my advice toward a firefighter like Aaron. If you're somebody who works as an EMS professional, then you've got experience I don't and that most other people won't either. But if you're an everyday person doing an everyday job like myself, a good purpose built tool in the glove box can be a lifesaver in my opinion.