It’s asymmetric. It is designed so that any person knowing a slavic language on a high level or natively could understand whatever is being said. However there’s a catch in that the same things are called different words in different groups of slavic languages. For instance “loše” would mean “bad” (adverb), but it’s a word only used in southern slavic languages (serbian, macedonian, etc). Maybe western slavs would understand it, but eastern slavs would not. In cases like these the writer or the speaker needs to cycle through a couple of alternatives to find what makes sense. But it is guaranteed that you will be able to convey a thought once you find the combination of words that the listener knows from the slavic language that they speak.
So in that regard, you could say the comprehension rate is somewhere between 80% and 100%, depending on the choice of words, without having studied the language.
However as you pointed out it requires some education to be able to actually speak it, as well as familiarity with the generic tendencies in slavic languages. For example a skilled user of interslavic would make no mistake in choosing the wrong words because they’d be familiar with the slavic group their listener operates in. However that doesn’t work for the general public unless you’re willing to repeat some of the words.
There’s a decent sized community of the speakers and the learners of interslavic, but it’s not very big. I think you could compare it to the wester online go community in english only.
Is anyone interested in the phenomena of idioglossia, or more commonly known as cryptophasia between twins?
I still have a notebook from my childhood with a codebook designed to record the secret lauguage between me and my brother. And I can still remember part of it, but some are completely lost to me. Some part of it cannot be written, since it is not pure vocal, but mixed with secret rituals and gestures required both of us to participate. Even as grown ups, we are still able to communicate with each other none verbally to a degree.
Language evolution might have been very rapid thanks to the easily incorporated constructive components that can artificially create communication barriers. (look at how fast Internet creates/shorten phases, incorporating symbols, non verbal meme components into them)
Please share! 
I can only speak English and Chinese, all fluent and I am learning Japanese.
Amazingly I consider myself horrific with languages.
I’m completely fluent in English, having had a post-graduate reading level since the 7th grade.
I have a middle level fluency in Esperanto. It is the only other language that I consider I can competently read in, or hold any sort of conversation. From here it gets much worse.
I have a very beginning level of fluency in American Sign Language. My mother, during my childhood, was head nurse at the NYS School for the Deaf, which is where the initial exposure happened.
I took three years of French in High School - the most difficult class I ever attempted. I learned (or retained) enough to use a French source during my graduate work decades later. Freaked out my advisor.
I know just a few common words of Japanese, basically garnered from my Shogi studies. I remember being on the Ithaca College Campus decades ago and seeing a bus with large Kanji lettering. “Golden…Horse,” I read. And then in much smaller English lettering under it which I didn’t see initially, “Golden Horse Bus Lines.” I was over the moon for the rest of the day.
I speak propa england
My native language are Bahasa Palembang a local dialect) and Bahasa Indonesia.
Rather fluent in English.
Know couple things in Mandarin.
Trying to learn Kawi (script of Majapahit kingdom)
In addition to these, I have started studying Russian for my second university course. I don’t have a good level yet, but I understand and can say simple things and I will improve 