I'm going with a solid 8, although I found the whole abandoned colony reveal to be a bit far fetched, after hoping for a fascinating new species. But the many character moments in the episode more than made up for that let down, including Saru's performance as captain upholding Federation ideals; it felt a bit like TNG to me, and that's not a bad thing. The Adira-Stamets interplay was deftly handled and fun, and I'm very interested to see where her (their) Trill story line goes. Wonder how big that tree was 900 years ago...