They are horrible, to be honest. You have to choose the aperture first, then screw on the rings, so you can't change the aperture while shooting. Also you have to be very close to the object and you can't auto focus. On top of that it is very very dark when you look through the viewfinder so you can't see anything. Today I tried them again and I was very frustrated with it, because I couldn't make even one good photo with it. So you'd better buy a real macro lens or rings with which you can auto focus. End of rant.
Ah. You might want to try the reverse lens trick. It's worked for me--and I've been able to take some fairly decent shots like that. It's pretty simple, and you can buy a reversing ring (which I highly recommend for sanity reasons) for only like $12ish.