I don't think there's any question at this point that Spanner is really cool. The vast majority of cloud databases out there (even brand-new ones like Azure's Cosmos) are still implicitly trying to convince you that not having ACID transactions is something that won't matter very much, and that scale is by far more important, which isn't true. Google hasn't taken that approach with Spanner, and it's a breath of fresh air.
I'd throw one downside out there though: lock in. While queries use standard SQL syntax [1], all write operations are performed via a very non-standard GRPC API [2]. I think the team certainly considered this and weighed the trade offs, but it does mean that after you're on Spanner, you're on Spanner forever.