It would have been good if it had been made all that usable. Many people were looking for a place to have all of their media in one. Smart TVs weren't the default like now, so they were trying to take the market from things like Roku and Apple TV and such. Still could have been useful if they had made it better than the crappy Smart TV interfaces (which wouldn't have taken much), but they never did, so it never really caught on.
I have an apple tv. If steam made an apple tv like steam box that played games, watched movies and tv shows and youtube id get one. I use linux so that thing doesnt exist and Id be pumped. I know many millenials wouldnt mind that for a tv