If you're referring to the screw-on type that goes in front of your kit lens, save your money. They're good for a door stop, paper weight, target practice....
I have one of those thingys that screw on the front of the lens to give a wider angle, also have one that magnifies the image, both are junk. If you want to go wide with the lens you have you can take a bunch of shots up, down and across and photomerge them or you can get a wide angle lens.
This was taken using a canon 350D and an 18-55mm lens. It's a total of about 24 shots...
One came with my t3i kit....here are a couple of pics to show you...if you are all the way zoomed out it will have the black shadows. It came in handy though, when I needed to take pics of all the jeeps.