Debian Stretch was officially released on Saturday, and I've built a new Stretch base image for VPS use in the WMF cloud. All projects should now see an image type of 'debian-9.0-stretch' available when creating new instances.
Puppet will set up new Stretch instances just fine, and we've tested and tuned up several of the most frequently-used optional puppet classes so that they apply properly on Stretch. Stretch is /fairly/ similar to Jessie, so I'd expect most puppet classes that apply properly on Jessie to work on Stretch as well, but I'm always interested in the exceptions -- If you find one, please open a phabricator ticket.
The WMF and the Cloud team is committed to long-term support of this distribution. If you are starting a new project or rebuilding a VM you should start with Stretch to ensure the longest possible life for your work.