The experience is weird - when I first went to Ukraine I couldn't understand almost anything. After a few days it suddenly "clicked" and I realized how the most common sounds and word endings relate between Polish and Ukrainian - and since that moment I basically got all the words with the same roots for free (which is like half the language). It also gave me Belarusian as a side effect :)
Of course it's not actual language speaking, I'm just understanding every other word and connecting the dots.
With Slovak it was even quicker, but somehow it hasn't given me Czech - their pronuciation is just too weird, despite the fact it's almost the same in writing :).
I'm not sure Interslavic provides much value. For me it's basically Slovak, so if you come from East Slavic language you'd probably get the same benefit spending that time learning some Slovak, and you'd then know some actual language instead of an artificial one.