If you're already on windows 7 there aren't that many reasons to upgrade.
But there aren't really any reasons not to either imo. Win 8 is mostly a further optimized version of 7 with some extras.
What some people go apeshit about is the new UI. Just give it a fair chance for a while, if you don't like it get one of those UI mods like start8 to get rid of it.
Anyway, personally I like it. It's stable, fast, supposedly more secure, faster boots and new task manager are nice. Helps that it's so cheap too.
I don't consider the UI an improvement, but for me it's not worse either, it's just different way of doing things. I'm not impressed by the way metro apps are integrated though. I don't know why anyone would ever use one on a desktop so they're mostly useless to me.
Took me a day to get used to the UI and set everything up but I can do everything at least as fast as in 7.