OK, I've just tried upgrading from 2.5.2 to 2.5.5, and it didn't go well!
I managed to run the installer, and it got me to the confirmation screen, where I deleted the installer, I can access the login page, and enter my details, and it starts to login in then sits on this url
Nothing happens (in Safari) and in Chrome, it says 500 Internal server error.
As this didn't work, I removed all files and uploaded my backup, and now I'm still getting the same!
$debug = 1; in admin.php, and now get the following error on the admin page.
OK, so went into database and had a look at the exp_security_hashes table, and I had a field titles session_id, I changed this to ip_address and it seems as though its now up and working.
Will keep an eye out and see what happens!