i've got some extension tubes, they are really hard to use. never used macro lenses, but i'd imagine it's more point-and-shoot than the extension tubes.
auto focus range becomes tiny, so you really have to use the zoom dial as manual focus. then there's the problem of trying to find that insect with a completely blurry view finder.
if you've got the money, go for the lens, it is usually a brilliant all round lens.