Well, personally, I don't bank on getting the green light. I know of only a few lights that have the sensors. I'm sure there are more out there than I realize. If I'm approaching the light and it turns green, I proceed through it [still clearing the intersection as always]. If it doesn't give me the green... then it's business as usual. Stop/clear/proceed through.
I'm not the only one going to a call a lot of the time too. Another emergency vehicle might be approaching the same intersection from a different direction. If the system hears them first, they're going to get the green, and I'm still going to have to stop.
It is just another tool that will [a majority of the time] facilitate the response of the emergency vehicle. It is one of those things you don't even think about, but is out there.