Unfortunately I don't have any upgrade experience. Nor to I have any install experience. Yet :). I'm in the process of doing that now. From reading the docs though, upgrading looks pretty straight-forward and easy to do. The only thing that I can think of, is that the upgrade doc doesn't yet mention TLS; the draft TLS docs say that you must set Mattermost to bind to low ports each time you upgrade
If you want feedback on the install experience, I'll gladly do that, probably in the form of a pull request on the install docs. I don't have a lot of time available for these kind of projects right now so it'll take a while, plus I want to use MySQL on Ubuntu 14.04 LTS, and the docs currently only cover PostgreSQL.