I have tried to use my 18-105mm with a Raynox DCR-250. It does have some slight vignetting at 105mm. If you don't mind doing some cropping, it does serve as a good and cheap macro setup. But I personally prefer to use my 50mm F1.8 with CM-2000A(DCR-150+DCR-250), but do note that the working distant between the lens and the subject is only 1.5cm. Which also dictate that my chance with taking butterfly shots are very very little. Any other insects that are not too skittish are fine though.
Wayne900- I will get a 50mm F1.8 to try out if you already have a DCR-250. And even if it's not working well for you, 50mm F1.8 still serve as a very good portrait lens. If you are serious about macro, go ahead with your purchase on Tamron 90mm or 180mm(whichever fits your wallet)