Are these worth $20?

Yes. Anything is worth $20, that's quite low end for a set with sub.

These aren't anything special, but they are certainly better than your usual dirt-cheap stereo computer speakers.

Get a better PC speaker system that will change the sound experience with your gaming or online video.
I used Cambridge Soundworks, Digital desktop theater 5.1, Altec Lansing 5.1 speaker system.. before and now eventually settled for Yamaha HT receiver + Energy sub + speaker system- the whole gaming experience is way different after upgrade....

These are great for the office. Friday afternoon! Tine to pump some JAMZ!

I use these for a laptop that's mostly used as a desktop replacement. They're definitely worth the money, and sound as good as some stuff selling for $50 (even Logitech gear). Pleasant surprise.
Decent ratings at testfreaks
http://www.testfreaks.com/pc-speakers/logibaconls21/
I'd recommend something better for your primary-use desktop, especially if you're a gamer or using your PC for music playback, but you won't likely feel ripped off with these otherwise.