They differ in reading as well as they differ in comprension. Let me give you an example: My friend went to Bosnia to see her relatives (she goes anually). When she returned from Bosnia, she was overwhelmed by the fact that she learned so many new words. Bosnian is constantly being under influence of Muslims and there you go. I'm afraid I won't understand them should I ever go there. Another example is the fact that according to my unofficial survey, most of the people my age (18 or so, which is an important age group) and some of the other ages don't actually know the names of the months in Croatian.
True that many people don't know Croatian words for months. That's why in casual conversation, we either use international names or just number them.
Funny you should say that Bosnian is hard to understand. Few weeks ago, a Bosnian film "Go West" was shown in Belgrade, and it's producer Ahmed Imamović was a guest on Belgrade radio station B92. Of course, the film was not subtitled Bosnian->Serbian. They assumed everyone in Belgrade can understand Bosnian.
Imamović is from Sarajevo, he talked about the film, listeners were calling in and talking to him. There was no trouble in understanding at all. I listened to all of it, so it was part Bosnian, part Serbian, and I understood it all.
Also, just this morning, over my tea, I was reading Croatian weekly Feral Tribune (http://feral.mediaturtle.com/), and among other articles, which were all writen in Croatian, I read this: http://feral.mediaturtle.com/look/weekly1/article.tpl?IdLanguage=7&IdPub... article by Petar Luković, who is writing in Serbian (if I am not mistaken, he is from Serbia and lives there). I read most of this edition of Feral Tribune, and I understood both stuff in Croatian, and stuff written by the Serbian author. If Luković was not writing about politics in Serbia, I would probably never notice that I am reading two oh so different languages.
Some time ago, in Serbian daily Danas (www.danas.co.yu), which unfortunately doesn't keep archives from more than few days, I read an article from Croatian Novi List (http://www.novilist.hr/), which was in its original form, so - Croatian. Iirc (they often use Novi List's articles), this one was regarding arrest of Ante Gotovina.
And just moments ago, I wanted to check the result of basketball game between Partizan (Serbia) and Bosna ASA BHT (Bosnia), so I went to http://goodyear.adriaticbasket.com/. I wanted to get details of the game, as I am a fan and couldn't watch it, so I went to their discussion forum topic: http://forum.adriaticbasket.net/viewtopic.php?t=4794 ... where fans of both Bosna and Partizan were commenting the game as it was progressing - with usual fan teasing (ok, teasing is a weak word). Nobody called for a dictionary.
Cheers, Dejan
P.S. Partizan won, if anyone cares.