Experience as 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.