Some points I handpicked for you. Ignore the uninformed masses.
The Debian GNU/Linux FAQ
Chapter 3 Choosing a Debian distribution
3.1 Which Debian distribution (stable/testing/unstable) is better for me?
If you are a desktop user with a lot of experience in the operating system and do not mind facing the odd bug now and then, or even full system breakage, use unstable. It has all the latest and greatest software, and bugs are usually fixed swiftly.
3.1.5 Could you tell me whether to install stable, testing or unstable?
Testing has more up-to-date software than Stable, and it breaks less often than Unstable. But when it breaks, it might take a long time for things to get rectified. Sometimes this could be days and it could be months at times. It also does not have permanent security support.
Unstable has the latest software and changes a lot. Consequently, it can break at any point. However, fixes get rectified in many occasions in a couple of days and it always has the latest releases of software packaged for Debian.
3.1.6 You are talking about testing being broken. What do you mean by that?
Sometimes, a package might not be installable through package management tools. Sometimes, a package might not be available at all, maybe it was (temporarily) removed due to bugs or unmet dependencies. Sometimes, a package installs but does not behave in the proper way. When these things happen, the distribution is said to be broken (at least for this package).
3.1.7 Why is it that testing could be broken for months? Won’t the fixes introduced in unstable flow directly down into testing?
The bug fixes and improvements introduced in the unstable distribution trickle down to testing after a certain number of days. Let’s say this threshold is 5 days. The packages in unstable go into testing only when there are no RC-bugs reported against them. If there is a RC-bug filed against a package in unstable, it will not go into testing after the 5 days. The idea is that, if the package has any problems, it would be discovered by people using unstable and will be fixed before it enters testing. This keeps testing in a usable state for most of the time. Overall a brilliant concept, if you ask me. But things aren’t always that simple.
tldr: “The goal of the Debian project is to produce Stable”. Sid is essentially a rolling release, it’s nothing like Fedora Rawhide in the old days when was essentially a testbed for Red Hat but it’s not meant to be as smooth as, let’s say, Arch or Tumbleweed. Testing in the other hand isn’t merely some layer between Unstable and Stable, it’s part of a bigger project, Testing exists for the sake of Stable, not for the sake of Testing. Same logic applies to Unstable but you do achieve some level of “just works” when you’re just pushing all the latest software, after all Debian also has Experimental but you should still expect breakage when something truly major happens.