I am subscribed to quite a few podcasts and screencasts here and there. And I’ve come up with a very basic (albeit limited and you could even say irrational) way of determining which to keep listening and which to throw away immediately:
The quality of the material… and the voice of the speaker.
I’m not Pavarotti nor Alfredo Caruso, but some voices just irritate me. I just experienced this through the Heroku screencasts; the guy’s voice is not really nice (at all), kind of creepy even, hard to follow, I don’t know how to describe it. It is annoying to follow a 10-minute presentation like this; really, I’m sorry, but that’s how I felt it, even if his service seems really interesting and I might even try it in the future.
Compare now with Ryan Bates of Railscasts: his voice is adapted, serious yet young, with the right pitch and speed. It makes following the explanations easy, moreover taking into account that I’m not a native English speaker. The Railscasts are a perfect example of what I like in podcasts and screencasts: short descriptions (15 min max) of extremely useful features, with practical uses and with some background as well to get the idea. The site (and Ryan) is absolutely brilliant.
As I said, is a purely subjective point of view, but that’s (one) of the criteria I use to decide whether to keep listening to a podcast / screencast or not. The other being the contents, of course; throw in a nice voice spitting nonsense and you won’t have much better luck than the Heroku guy.
The notable exception to this rule must be obviously David Heinemeier Hansson; his first videos showing how to do a weblog in Rails in 15 minutes are just insane; the guy’s voice is really awful, too highly pitched and somehow disturbing. But the stuff he showed was great, and I stuck with that instead :)