Experience as a background actor?

I was wondering if anyone can share their experience as a background
actor? I was invited to go as an extra this coming Monday, not even
sure what film they're filming here in Toronto, and they're not sure
about the length of filming either, but I guess that's normal. I would
go if I was free that day, but problem is I have an important class
lol. So I was wondering if anyone could share their experience (as it
applies to Canada... read a lot of American version stories already),
is it worth it? Payment is not a concern at all, I don't really care
about the money earned, more into how the experience is like. Not
really into acting, and don't have an fetish for trying to get close
to celebrities, but I do want to see how a "real" filming process
works.
Thanks.