The 100mm macro is a moderately large lens, and its design and construction will look instantly familiar to anyone versed in Canon's recent 'L' series designs. The exterior of the barrel is made from high quality plastic, which helps keep weight down, and the mount is of course metal; there's also dust and moisture sealing (with a protective seal around the lens mount). All-in-all it feels perfectly well-built, with no creaks or 'give' to the construction; but in truth little different from its non-L unstabilized sibling (aside from the sealing of course).
The controls are well laid-out and clear - the three control switches on the side are positive yet difficult to knock accidentally, and the focus ring is broad and smooth in operation. The plain cylindrical area close to the mount is the attachment point for the optional Tripod Mount Ring D; this is a useful accessory that avoids the radical changes in position and balance associated with flipping the camera to portrait orientation on a conventional tripod head, but one that's not especially cheap.
On the camera
The 100mm macro is probably best-matched to Canon's mid to high-end DSLRs, such as the 5D Mark II. It's just a touch front-heavy on entry-level models like the 500D, but still perfectly usable. The narrow cylindrical shape to an extent gives it the impression of being larger than it actually is - it's still distinctly smaller and lighter than the 24-70mm F2.8L, for example.
The lens uses Canon's familiar ring-type ultrasonic motor for focusing, which is fast, positive and near-silent in operation. Overall it feels a bit snappier and more decisive than the older 100mm F2.8 USM macro, addressing one of the few criticisms of that lens. As always, though, it must be noted that focus speed and accuracy is dependent upon a number of variables, including the camera body used, subject contrast, and light levels.
The manual focus experience is critically important for a macro lens - most serious closeup work is done tripod mounted, with autofocus not an option. Fortunately the 100mm F2.8 L IS USM macro works well in this regard - the gearing of the focus ring is, not surprisingly, well optimized for macro shooting, and makes achieving precise focus a breeze, especially when using magnified live view as a focusing aid. There is, however, one downside to this, which is that the travel through the 'portrait range' (ca. 1-3m) is distinctly short. This makes really accurate manual focus at such distances much more tricky.
Three Canon EF 100mm F2.8 macro lenses compared
Here we see the three 100mm F2.8 Macro lenses Canon has made for the EOS system side by side. They've grown progressively larger with each generation, and the filter thread has expanded accordingly, from 52mm via 58mm to 67mm. The original 1990 version on the left looks the most compact in this view, but unlike the other two extends considerably on focusing. It's also the only one with a metal body - the other two have plastic outer barrels. It was replaced by the 100mm F2.8 USM macro in 2000 (the first internal-focusing macro lens capable of 1:1 magnification), which added an ultrasonic motor for near-silent autofocusing.
The latest iteration has grown only a little compared to the older lens, and despite the addition of image stabilization is less than an ounce (or just 25g) heavier. But the red ring around the barrel which designates that it's a member of Canon's top-end 'L' series adds a hefty premium to the price. Because of this it doesn't replace the older lens, but rather sits alongside it in Canon's formidable short-telephoto lineup.
Lens body elements
Reported aperture vs focal length
The lens allows apertures from F2.8 to F32 to be selected.