by Howard Miller.
Once you have your installation under git, you can upgrade to the latest version of the same branch by a process this difficult...
git pull
(go to Site administration > notifications)
Provided you don't have loads of modifications or optional plugins then upgrading to a different branch is not much harder. Probably...
git checkout -t origin/MOODLE_27_STABLE
In fact, even if you do have optional plugins that might be all you need (but it depends how you set it all up).
If you are a developer - and by that I mean anybody who changes core code - then it's a no brainer. If you don't use version control you are doing it wrong.
Need more convincing?
NB: Always take a backup before doing any of the above!!