I've used Paper a number of times, starting from long before Notion was a thing, having been a happy Etherpad and Hackpad user in the past. Somehow Paper managed to inspire less in my collaborators than either the latter services (where perhaps plain text was the factor) or than Notion (the reaction of almost everyone I've shown it to has been amazement and excitement).
Since this is the second time I'm mentioning Notion here, I'll reiterate that I'm not affiliated - just a happy customer.