I do. Since 2009, I've been a lead administrator on a site. The site has gone through a lot of changes and even died a few times. The previous owner got busy with college and such, so I ended up owning it and rebooting it with a few friends/old members in early 2014.
It definitely takes a lot of time to organize things, but it can be a lot of fun if the community is made up of cool people. Being good friends with your staff is super important too since you want to avoid drama. I think of them as family.
I'm picky about forum software and still unsettled with my current choice—Vanilla Forums. I really dislike my site's current theme, but I've been too busy with work to be able to rewrite the whole thing. On that note, my favorite forum theme insofar has been this forum's. We're looking to start from scratch—including the forum software—when we're all free. More time consuming than hard.
So yeah, forums require a lot of time.
Here's a question for those of you that don't own one: If you had to make one, what would be its focus?