testing is the only one of those that's remotely usable, and it gets frozen as soon as they start thinking about releasing -- etch was frozen for five months, lenny for seven. Sid / unstable is generally frozen whenever testing is, is still really laggardly normally, and is constantly broken anyway. I've never gotten anything useful done with Experimental.

At least volatile has been around for a few years, so people aren't fucked on tzdata and such because of bullshit policy, though I think it's still off by default.