If we're talking human usability a dash(-) is a single keystroke while an underscore (_) is a shifted keystroke. I find the former faster to type when actually entering URLs in the address bar.
Indeed, but I can't remember any occasion I've ever manually typed the whole URL like "stackoverflow.com/questions/1234567/test-here-is-the-title".
One can't be both descriptive and typing-friendly at the same time, so there's always room for URL shorteners and shortcuts ("/questions/1234567", but something like "/~1234567" is better).
