My perspective from working both inside and outside of Google:
The external spanner documentation doesn’t seem as good as the internal documentation, in my opinion. Because it’s not generally well known outside of G, they ought to do a better job explaining it and its benefits. It truly is magical technology but you have to be a database nerd to see why.
It’s also pretty expensive and because you generally need to rewrite your applications to work with it, there is a degree of lockin. So taking on Spanner is a risky proposition - if your prices get hiked or it starts costing more than you want, you’ll have to spend even more time and money migrating off it. Spanner’s advantages over other DBs (trying to “solve” the CAP theorem) then become a curse, because it’s hard to find any other DB that gives you horizontal scaling, ACID, and high availability out of the box, and you might have to solve those problems yourself/redesign the rest of your system.
Personally I would consider using Cloud Spanner, but I wouldn’t bet my business on it.