I have probably the same understanding of serbian. I used to read a lot of science fiction literature translated into it. Not to mention that I learnt cyrillic script in elementary school in the 1980's when the language was still officially called "serbo-croatian". But there are significant differences: for example serbian people don't understand kaykavian and refuse to. There are a lot of jokes on the theme in old yugoslavian tv shows. Kaykavian is part of croatian language.