OK?

I would add that I have two macros - a 150 and a 180. One day I went out to take some shots of high schoolers playing lacrosse, thinking I had my 80-200 F2.8 in the bag. I didn't, I had the 180 macro. Well the best lens is generally the one you have with you, so I shot 300 or so images. This lens does not easily manually focus, you have to grab the focus ring and slide it forward to release it from the camera's focus drive, so manual focus on the fly was out of the question. It was a bright day, so I used a smaller lens opening - F8 to F11. I would say that about 250 or so had no focus errors, partly because of the greater dof with the smaller apertures. But, the images were a bit soft, and the backgrounds distracting. Otherwise, I got the images. I would not have had those issues with my 80-200.

Both magnificent photos Liam, but my comment was actually a bit facetious.
I was making the point that the lenses are "different horses for different courses" and suggesting that you won't find any 24-105 shots in the True Macro forum.
You don't compare primes to zooms and then wonder why you would buy a 100mm macro instead of a 24-105mm, well you do, but its more about the macro capability than the focal length.
The good thing is, even though I was being facetious, you example post showed the point perfectly with great photos.
How's those eyeballs on the first one????

Yes, many choices. Here are my thoughts, the 100 mm Macro is superb. Very sharp pictures. But that is not all, at 2.8 aperture, it is good in lower light. Second, and very importantly Image Stabilization. I use the 100 mm macro for a lot of outdoor pictures of kids in the backyard, when I go to a flower show. As for primes, I own the Sigma 35mm, 1.4. Great for indoors, low light, wide angle for family pictures. I also own the Canon 50 mm 1.4, also good for indoors, but I find that the 35 mm is better due to the wider angle.

I would not hesitate at all to buy the Canon 100mm macro. It is just great.