Recs.
Updated
Arch Linux Testing contains packages that are candidates for the core or extra repositories.
Specs
Pros
Cons
Con Needless breakage
Arch is a lot more stable than people usually give it credit for. Yes, it can break, so don't install it unless you're pretty sure you can fix things if they break, even if they probably won't.
Arch Testing is literally meant for if you're developing packages. Find a bug? That's because you're using beta software. You're supposed to report that bug, look over the code, create a pull request, and donate your time and energy to a project. What I'm saying is that Arch Stable breaks much more rarely than people give it credit for and Arch Testing breaks exactly as often as people say Arch Stable does.
Arch Testing is GREAT if you're trying to help make open source software in general better, or if you have your own project that you want to test against other upcoming software releases so you can fix it. If you're just a regular user, just... don't. You will be more than happy with regular Arch and it probably won't break.