Russia, Ukraine,Belarus, Bulgaria, Macedonia use the Cyrillic. Serbia and Montenegro use Cyrillic(mostly) and Latin. Any other countries use Latin alphabet only.
Polish, Czech, Slovakian, Ukrainian languages are intelligible partially, so people can understand many things.
Russian is another story--it's rather less intelligible with other Slavic languages because of Turkic(more) and Finno-Ugric(less) influence on this language.. Slovenian language is also differ from the other Slavic languages though it has similarities with Czech, Slovakian and Serbo-Croatian languages. Speaking of very old and archaic Baltic-- it considered that Baltic and Slavic languages basically belong to the same dialect group but as russian I can say it has nothing in common with Russian at all. Though I believe Poles can understand some things in this languages.
And by the way, Bulgarian is NOT Slavs--its Turkic tribe from Caspian-Volga region assimilated with Slavs and Thracians. The other branch of this tribe is Volga Tatars in Russia.
Concerning the other things in this thread, in short I can say that Flankerl is almost totally right about everything.