The topic of mutual intelligibility in Slavic languages is a bit difficult For example Bulgarian is routinely regarded as a language most distant from the Slavic type and very difficult to understand for most Slavs In Czech there even exists a saying speaking like a Bulgarian for unintelligible speech Yet the difficulties in understanding lie mainly in grammar the nonexistence of wordendings the use of articles You can listen to a Bulgarian not to understand a word and doubt if you are actually listening to an IndoEuropean language  and then you see a Bulgarian text and you are surprised that you can read it quite with ease So the difference in understanding spoken language and written language can be huge  As for the difference between CzechSlovak and Polish I must agree with the Polish contributor above that a fluent conversation is not possible Yet you can certainly understand a lot and conduct some conversation if you try to be patient So a limited intelligibility would be a more correct term As for written Polish Czechs and Slovaks shouldnt have any difficulties with a general understanding Polish text although quite a few words especially of technical sort may seem obscure and untintelligible However the closest Slavic language to Czech after Slovak is Lusitan Sorbian especially Upper Sorbian I dont have any personal experience with this language but from the written texts I saw I think that its resemblance to Czech can be compared to East Slovakian dialects which means that you can understand some passages easily but subsequently you can be lost due to accumulation of weirdly sounding words