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 new photography 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.