[xGarnetx]

I think "amigo" is a very common word in Spain. But, maybe, only in some contexts. I mean, you can say "I'm going out with my friends" "he is my friend" or I like to talk with friends" or anything, but, for example, if you are truly friends you won't say "Thank you, my friend". In my opinion, it is a bit cold, it seems you aren't very close, I would rather use his/her name. So, "amigo" is used as a noun or and adjective, but not to refer to the person you are talking with.
I hope this is the kind of answer you wanted, you can ask again if it's not ^^U


About the differences:
In my opinion, the differences between american spanish and european spanish are bigger than differences between english in Uk, America, or Australia.
I can understand (maybe not absolutly every single word, but the meaning) anyone writting in english, even though I only study british english, but, if you are studying spanish, it would be difficult for you to understand some phrases in latin spanish without a good online dictionary. Anyway, it is not a problem of american people, this kind of problems you would find them in latin spanish slang, which is much different from spanish slang, and what makes it more difficult, it is different in every country of america.