I've heard many mixed things on that. I think adults liked it much more, but kids a little less because it had a "downer" of an ending. I think that is what really happened then and still now. I remember when I was younger and first saw the OT in the late 80's I preferred the original and ROTJ over TESB. But as I got older, that changed.
My father said that most of his friends loved ESB the best from the get go. He said the "I am your father" twist was so out of the blue, they could not stop talking about it. It fleshed out the characters and made you like them more.
Though I can't speak for everyone. I know now critics usually agree the ESB is still the strongest one in terms of everything else. I think it was still universally loved, because it was Star Wars still and very exciting, I think the ending just threw some off, many became so attached to Han my dad was saying they really at the time had no idea if he was going to make it or not.