I've got a set, I don't use them often, but I don't shoot a lot of macro or close up.
As mentioned, they to tend to degrade the image a fair bit as you get to the edges, but with a lot of close up photography, you are working with very shallow DOF, so you may not be concerned with the sharpness of the edges as they won't be in focus anyway.
You can even stack these filters for greater magnification, but you also compound the problems.
They are fun to play with, and certainly a cheaper option than a true macro lens. If you find that you really like using them, then consider upgrading. If you don't use them much, then you didn't waste $500 on a macro lens.