What kind of software is available in that source tree ? Also what is the best way to see what kind of software is available in a repository ? I have always reluctant to enable it because I didn't want to wind up with an unstable system.
Backports are basically just untested software. It won't hurt, it won't cause a problem. (At least it never caused to me on productive machines). However, you can just open the packages.ubuntu.com website, check the backports. Fetch what you want an upload it to your PPA. (You will learn about this, there are so many good documents on Launchpad. But if you are interested , I can write it down in resolution.)
So basically they are just newer software, but they don't test it out that much. Uhm.. like on OpenSUSE a Mozilla upgrade from the non-SUSE repo breaks the KDE integration. Like that. But on Ubuntu never seen something like that.
About repos: You should NOT enable the testing updates. That caused my system to break down already. So yeah thats the only repo which you should leave it as it is.
Also, using backports is still more supported than the PPA method. PPAs belong to people. People can put there anything, can mess up their packages, Launchpad can't check the packages that thoroughly. (Like I messed up the changelog not once and the packages uploaded just fine.) So yeah beware of PPAs but they are also great stuff. (You can also ask an expert/IRC or just fetch thep package and check for yourself before installing it.)