Never.
Did use it when I 1st started the game to see how it worked. Couldn't stand it. In the short time I tried it tho, I became used to hitting the vats key as a 'panic' button and it took me a while to get out of that habit. After that...except for a couple times when I wanted screenshots, haven't touched it at all, period, since.
For me it's partially about vats being too much like a win button (not always, but often), but even more, it's completely immersion-breaking.
Me too, I still have trouble trying to stay away from the life alert button.
"AAAAH, Cazadores!? Where did they come from?!" - Whenever I see a cazadore.
I don't blame anyone for using VATS on these little bastards, you need something to bring them down a notch.