In a nutshell, RiffTrax are the MST3K guys ripping on recent movies you have actually heard of. Buying two of them now has not made me an expert by any means, but it has let me draw a conclusion.
They’re really good. And they’re definitely worth the $1-$4 each. You obtain the DVD on your own (rent, buy, steal, whatever), then go to RiffTrax.com to download an MP3 file of the commentary. If you’re watching the movie on your computer, you can have the commentary sync up automatically, otherwise you start them at the same time and there are periodic checks to make sure they’re in sync. (Every 20 minutes or so, a voice on the RiffTrax MP3 says a line of dialog at the same time as a character in the movie. If one is ahead of the other, pause it for a few seconds to let the slow one catch up.)
Humorwise, RiffTrax are on par with MST3K: there are 5-6 truly gut-busting observations per movie, several dozen really good chuckles, and a fair number of “yeah I guess that’s funny” moments. It feels like renting some very funny friends to rip on movies for the evening. I can only speak for “Independence Day” and “The Matrix” (so far), but the RiffTrax definitely improved on the original. The website’s selection isn’t everything you’d want to watch, but there are plenty of movies to pick from and you won’t run out anytime soon.
Another plus: instead of paying to download the file only once, the files are added to your account. You can download them multiple times, to multiple computers. This makes a huge difference if, say, you watch a movie on your computer and then later decide to go sit in the living room and watch it through a different device there. Or you can load the RiffTrax onto an MP3 player with headphones so nobody else knows why you keep laughing. Whatever floats your boat.
I give RiffTrax an enthusiastic two thumbs up. Pricewise they’re right on, they are funny, and they only take a few minutes to download and set up (assuming you have a reasonable Internet connection) so you don’t have to pick a movie hours ahead of when your friends come over. Definitely worth a try.