Stackdriver is great because it is built-in, so its trivial to get started. It is not (yet) a very sophisticated data visualization and alerting tool though.
We are starting to look into using Datadog, since it permits you to create graphs and alerts on queries over custom metrics, which we are starting to need.
In both cases: Getting *custom* metrics from App Engine code into Stackdriver or Datadog is a small pain. We've written a small custom "stats aggregator" that reads from a pull queue and sends to the appropriate API. Our application then writes a task to that queue at the end of each request, with the stats that were updated.