I've got a Raynox DC-250, you can get some excellent super-macro images with it but it's tricky... you're down to a few mm working distance usually and depending on what lens you use with it you can get some very heavy vignetting. These, diopters and reversing rings are all cheap and cheerful options that can net good results, but they're all finicky and none of them can truly replace a proper 1:1 macro lens.