I like Google Datastore alot - I think it is the best of the serverless databases at the moment, assuming you are wanting to build "normal" crud style web applications to store stuff and query it in a way that is reasonably familiar to most programmers. It's great and well thought out.
The weird thing about Google Cloud Datastore is that its essentially got no real search functionality.
I say "weird" because, well, it's Google and you'd think that the search thing would pervade everything.
I recently had need of a serverless database, and I'm a huge fan of AWS and I use AWS Lambda but AWS really lets its side down with its serverless databases DynamoDB and SimpleDB, neither of which, in my opinion, are usable for "ordinary" applications.
So in the end I implemented Google Cloud Datastore in my AWS Lambda functions.
After a couple of days I bailed out on the strategy and installed Postgres on an instance and now I'm using that.
There were several reasons to finally go (back) to Postgres. First, the Google Cloud SDK for JavaScript was huge which was a nightmare for uploading AWS function zip code. Second it made me really nervous that I couldn't do any sort of LIKE or search query, which I need in my application. Third the documentation for DataStore was good but not great and there wasn't alot of third party stuff written on it. Given it was early in my project I decided to retreat to the safety of Postgres to avoid buyer regret.
I still feel that there is a real need for better serverless databases - Google Cloud Datastore has my vote as best so far but not ready yet for the sorts of thing I want to build.