Western Slavic Languages

Most of us know that this includes Serbian, Croatian, Slovenian, and Bosnian. I know that S/C/B are very closely related, but the question is how related are they? Are they close enough that if I learned one would I understand the other ones rather well, or are they just mutually intelligible, and learning one would get me by in the other countries? If so, which one would be the best to learn. In an example, they say that Norwegian is the middle language, it shares vocabulary with Danish, but phonology with Swedish. How is it with West Slavic languages? I'd like to know, because learning one seems pretty attractive if it's 3 for 1.

Just for the record: Croatian, Serbian and Bosnian belong to the south slavic language group, not to the western one.
However, the differences between the three languages are very small, maybe even smaller than the differences between two German dialects. I have a Serbian friend who says he could understand Croatian and Bosnian without any problems. Slovenian is more distant though.