Hi there. I bought some of these a couple of years ago for use with an Olympus E-500 + 40-150mm lens. For the money I don't think you can grumble really and they certainly magnify. For less than £15.00 though I don't think anyone would expect them to give perfect results and, at least in my experience, they don't. I found them to make things a little soft whilst chromatic abberations are quite pronounced, and immediately noticeable.
For a few more quid you could get a Raynox lens and adaptor - I think the MSN-250 is the most popular? The adaptor works like a standard lens cover to clip onto whatever lens you are using and the close up lens screws into it. I haven't used the set that I bought in earnest yet, but initial results look very good - sharp with very little/no CA. I bought the RAYNOX CM-3500 MACRO EXPLORER KIT from digitaltoyshop @ £80.00. The RAYNOX DCR-250 is £40.00 - I think there is a 10% off code floating around for digitaltoyshop.

You may be able to find it cheaper on ebay.