The Pros are: they're cheap and small
The Cons are: glass quality not great, not great build either.
I recently went to RHS Wisley to shoot the butterflies, but have no macro lens. This is a problem since butterflies are rather small and tend to be just out of reach. For a half decent image, one needs to fill the screen. I took along my Nikon D4 with 70 - 200mm f2.8 VRII attached thinking I'd get closer - wrong. This lens is not really made for this and the closest focus distance wasn't nearly close enough even at 200mm, hence the macro attachments experiment.
So there I am with a normally superbly sharp lens and a pro camera body and I've stuck these very cheap bits of glass on the end in order to get closer..................needless to say I've seriously degraded my equipment, but not to the point of unacceptable, just a little too far from perfect for me.
Things are murkier, I had trouble with the condensation in the hothouse (which I would have done anyway), and these in no way replace a macro lens - just not enough detail.
Have look at my results and decide for yourself...............
110mm, f9, 1/50, ISO 6400, yes 6400!! High ISO required in order to bring up the shutter speed, which is still very low. Depth of field is so shallow at any wider that nothing in particular would be in focus apart from one little flower.
200mm, f9, 1/125, ISO 6400
Conclusion is that you'd be better off using an every day lens with a closer focus distance or shelling out loads of money for a real macro lens. The above images are way far from real "macro" in many ways but got me closer than I would have done. I'll get Gill to put up some proper macro shots and you'll see what I mean.
These are fun to have a try with, but far from ideal.