As with most things, Sean, you get what you pay for. There are plenty of cheap ones about that really just don't cut it. Equally, Canon's 250D and 500D dioptres, along with some produced by B&W, are excellent....the Raynox offerings fall somewhere in the middle. You might also consider an extension tube or two if this will achieve the same thing for you (you don't say why you want them)
Bob