Are you talking about the ones that go onto the front of a lens...usually on a 'point & shoot' type camera?
I haven't used them personally, but my opinion is that you get what you pay for...and as you mentioned, these are cheap. They will certainly degrade your image quality, which probably put them into the 'not worth it' category.
That being said, I can see that the wide angle ones might be fun and even useful. The telephoto ones aren't as useful IMO. You would probably be better off just taking a shot with just the camera and cropping it. A wide angle one, on the other hand, gives you a view that you just can't get on the camera.