I love macro photography and I think there's one or two other things to look at. First of all the close up lenses are normally uncorrected 1-4 dioptre lenses but there are better corrected multi element add on lenses available. I use a 10 dioptre with my Canon 100mm macro lens.

Another add on I'm very fond of is the reversing ring. this means I can screw my 40mm Konica standard lens with the aperture wide open onto my Canon macro lens. This gives very close high quality lenses.

As far as other lenses are concerned - just give it a go. Many macro subjects such as insects are not flat so it doens't really matter if the lens is not flat field or the corners are a bit blurred - makes no difference to the important detail of the picture.

One last ppoint it's often better to use manual focussing and move the camera backwards anf forwards rather than rely on auto focus. I've post some stuff about this on my site

Yes macro is full of wonder