For someone who speaks English as their native language, it is quite difficult, but still not as hard as Arabic or Chinese. It's still one of the hardest languages in Europe alongside Hungarian and Estonian. But I guess for a native Estonian it shouldn't be too hard, since the languages are quite similar.
I think one of the things that makes it stand out from most languages is the fact that basically everyone is speaking some sort of slang. Almost all words can be cut into shorter slang words, which you can't really learn unless you listen to people speaking Finnish. People will still clearly understand you if you speak it normally in the more formal way, but almost no native person casually speaks like that. :)