As a Slovak, Interslavic feels easier to read and understand, though I can understand Slovio as well.
From my experience, Slovak speakers can understand other Slavic languages pretty well: Czech is basically our second native language (although for some reason it seems Czechs don't understand Slovak quite to the same degree), Polish, Slovenian and Croatian are understandable as well. Russian is a lot harder, even when written in Latin. I don't have experience with Ukrainian.
From my experience, Slovak speakers can understand other Slavic languages pretty well: Czech is basically our second native language (although for some reason it seems Czechs don't understand Slovak quite to the same degree), Polish, Slovenian and Croatian are understandable as well. Russian is a lot harder, even when written in Latin. I don't have experience with Ukrainian.