Yeah, there were many such attempts in Slavic world. It gives you uncomfortable 80% understanding, while just speaking your own languages gives you like 60% between most pairs. Not good enough to make people use it.

From my personal experience as a native Slavic speaker in other countries, this doesn't work well across groups - i.e. if my language is East Slavic, and my interlocutor speaks West or South Slavic, we can't really get far; certainly nowhere even close to 60%.

But, yes, with English being the de facto standard, these more localized attempts are becoming redundant - you want to know English anyway because you want to talk to more people, and once you know it, you might as well use it to talk even to people whose languages are related to yours.

I'm Polish, and with Slovaks, Belarussians, Ukrainians it's more than 60%. With Czechs it's maybe 50% cause of their pronounciation, with Russians it's maybe 40-50%, because they have weird word roots.

Maybe it's because I'm vaguely aware of the phonetic changes between east and west Slavic languages (ić - it, ska - skaja, etc), because I've heard some Russian and Ukrainian on street markets in 90s.

Haven't had much experience with other Slavic speakers, but Serbian sounds quite close to western Slavic from the songs I've heard.

There is also Interslavic that continues from Old Church Slavonic: http://steen.free.fr/interslavic/

I natively know Polish and I know some very little Russian back from high school and Interslavic is more clear to me than Slovio.

I'm interested in linguistics so during one CS conference in Zagreb I asked a few Czechs, Croats, Slovaks, Slovenians, one Belarusian (native in Russian actually but he knew some Belarusian) and back in Poland a few Poles who know only Polish and the results were quite mixed.

It really might be better to just treat this as a curio and speak your own (especially in your own West/East/South Slavic language subgroup) and hope for the best. Or just speak English...