Hello, I use SVN to install and upgrade piwik. It's fast and efficient and "just works".
For example, after checking out http://dev.piwik.org/svn/tags/0.5.4, if I want to upgrade to 0.5.5, I just do:
svn switch http://dev.piwik.org/svn/tags/0.5.5
...except that this time, it didn't work. svn fails and says that it couldn't add the 'tests/pear' directory : "an object with the same name already exists." (or something along those lines).
I can't upgrade until this is fixed... I tried svn revert tests* and svn update and the switch again, no luck.
tests/pear (and tests/TestFramework) are external dependencies.
Backup your config/config.ini.php, and do a fresh checkout (svn co).