If for flowers and products go 100mm 2.8L IS if can afford it, it also doubles as an awesome non-macro prime with IS and has amazing quality. If for bugs, do exactly what this guy says, cheap and better suited according to everything I've read.
An Introduction to High-Magnification Macro Photography
Someone here a while back suggested something similar when I asked about putting the 100mm on ext tubes, told me to do what that guy above says to do, and I almost ignored the advice completely because of how bad I want the 100mm for it's variety of uses and amazing quality. But I'm glad I had time to think about it, because I am short on cash and my primary intent with macro is photographing insects ultra close, and for that the cheapo custom rigs are said to be much more effective according to all reports I've seen, unless possibly you can afford that MP-E thing + Flash and rail and so on. I will likely be back with first hand experience with a setup like the above in about a month give or take. So take everything I say here based on extensive reading and studying and ZERO first hand experience with macro photography.