Are "track days" worth it?
I think they are. The only problem is once you start down that slope, it gets pretty hard to stop unless you have a fierce amount of repellent to stop the track mod bug from biting you.
You can run your car in stock form, but at a minimum expect to go through tires (outside edges) and brake pads.
I do think that there is a world of benefit though in that you tend to see things differently when driving, at least I do. Instead of just seeing a corner, I think about turn ins, apexes, and such. I look further down the road than I have in the past to try to anticipate events. I look at experiences like this to build upon driving skills. You never know when the unexpected obstacle will present itself.