There's one big reason this 100mm lens is Canon's top macro lens: image stabilization. Getting in close enough for a 1:1 reproduction ratio exemplifies any camera movement, which is why it's so nice to have a stabilization system on a macro lens. The system is more than your basic IS too, it's a hybrid system cutting back on camera shake from multiple directions. The 100mm length is nice for macro work, putting you at least a foot from the subject, though some photographers prefer to be a bit farther. As an L-line lens, the design of this macro is fairly solid, with weather-sealing. Focus is quick--and quiet with the USM, so you're less likely to scare skittish bugs. Macro work tends to require manual focus more often than most other shots, and this lens offers a larger focus control with a rubber grip for easier use of that feature.
The Canon EF 100mm f/2.8L IS USM is also one of the top rated EF macro lenses for image quality, scoring high for sharpness. The lens does tend to be a bit more prone to vignetting at the widest apertures. With all the features, the lens is also a bit heavy for a prime--not obstructively so, but more than several comparable focal lengths. With all of the features, this lens will also cost you almost $1,000.