But then I kept seeing it pop up on lists of underrated movies so I finally gave it a chance.
It's really good, but it's hard to put into words why.
I think mostly it's not afraid to take spy movie conventions and twist them. Sometimes it does exactly what you expect it will but other times it will zig when you think it's going to zag. I'm sure the fact that I've basically seen every Bond movie made me appreciate Kingsman more than if I saw it in isolation. It definitely has a Bond undercurrent, but it's much more than that too.
There are a lot of little details I could pick out and examine but any in isolation seems too small to do so.
So I'll just say this... if you like spy movies you should watch Kingsman.