I personally think that while it's clearly great for security and possibly good for Firefox in general, it's also terrible for the extension ecosystem. The set of possible extensions is being reduced from "literally anything computable" to "what the API supports." Sure, (some) popular extensions are basically being grandfathered in by having APIs added just for them, but that doesn't help the extensions that aren't written yet and will now never be written.
Essentially, a major selling point of Firefox has been sacrificed in the hope that the increased security, better perf, and greater development velocity leads to greater marketshare. Well, I hope so, but I think the jury will be out on that for a while.
Other than that, Quantum seems great. I did lose all my tabs when it updated (boo - that's never happened before), and I think the whole-window-width address bar dropdown is bizarre and ugly, but other than that it's great. I suppose the only long-term concern is that default merger of the address bar and search bar - configurable options have a habit of eventually being dropped, and I think a world of omnibars is less usable and less privacy-conscious.