This has been a long time coming. The autoscaler has been an interesting, and possible the most interesting, part of AppEngine and IMHO has recently reached the point (in the last year-and-a-half) where I'd say I'd want to rely on it for production.

It wasn't always this way -- at one point we were having so many issues with AppEngine's autoscaling that we ended up in a meeting across from Ben Traynor and heard some of their engineers discussing post-mortems of scaling failures in gruesome detail. They've come a long way since then and AppEngine has really found its way from something of an internal science experiment to what I think is the best competitor for AWS' offerings.

FWIW, the "backend" or "basic" scaling module that was available for both AppEngine and GCE was so poor and unreliable that it made it nearly impossible to scale anything in a way other than manual. I honestly hope they deprecate this and roll the few features into the autoscaler.