In my experience they're not worth the trouble. And they can't beat just placing a board at the bottom of the goal for the ball to bounce off of and roll back out to you.
how would you set that up?
Basically I got a 1 foot by 2 foot piece of plywood and leaned it up against the base of my goal then I ran a chain around it to keep it attached to the goal so it wouldn't fall when a ball bounced off of it. Basically, make the shot, it'd bounce the ball straight back out towards the top of the key...of course, you still have to make the shot otherwise you're still going to have to rebound it yourself.
Edit: I can take a picture of it later and show you what I mean if that's helpful.
