Proper dedicated macro lenses are way better than the all purpose lenses with a "macro" mode so definitely go for one of those if you can afford it.
I have the Sigma 105ex and a Tamron 180mm which are both great, the Tamron provides a slightly greater working distance which is good for the bugs, but offers very little if any advantage for subjects like flowers.
After reading a substantial number of reviews I decided not to bother with the somewhat more expensive Canon lenses, they seem to be of a better build quality but don't seem to offer much better image quality. I wrote a quick report of my experience with the Tamron
here.