Long ago, I thought about learning Slovianski (one of a family of auxlangs meant for speakers of Slavic languages), purely because I thought it may give me the ability to understand (to some extent) multiple Slavic languages. I never did so, though. Note that Slovianski has been merged with other 'interslavic' conlangs into a single conlang project. I think the new conlang is just called 'Interslavic'. Also note, if you do try to look up one of these conlangs, most of the sites are being made by the guy who made Slovio, who constantly makes websites lampooning other Interslavic projects (most of them teach a slight modification of his auxlang Slovio, which is really just Esperanto with a Russian-based vocabulary).
I would like a more professional opinion on the usefulness of learning this language. I don't know much about the Slavic languages, other than their phoneme inventories and their huge number of inflections.
Would knowing one of these Interslavic conlangs actually let you understand some natural Slavic languages, or is it only really useful (or as useful as an Auxlang attempt can be) for native speakers of Slavic languages?