I’ve been using it for nearly two months now (beta). I like that it uses a lot less CPU.
It’s biggest draw, as always with Firefox, is how customizable it is. The addition of WebExtensions expands this. The customizablility leaves vanilla lacking.
Google, and Chrome more relevant to this discussion, tends towards crooked practices when a product gets significant market share. Some recent events with Chrome worry me deeply. I am hopeful that Quantum can regain some of Firefox’s support to prevent the growing monopoly.