I love IR, but as i have stated elsewhere I may be a bit weird. I think it is quite a challenging degree but not nightmarishly hard. It requires a lot of reading and getting to grips with the theory and some people can have trouble over the fact that there aren't realy any 'right' answers. (personaly thats one of the bits i like).
If your good at essay subjects IR shouldn't prove to problematic.
Just about any issue going on in the world at the mo could come under IR so you've got a vast number of things to focus your study, pretty muck anything that interests you.