I wasn't certain this lens would be a good value or choice for an APS-C camera. Having recently purchased a 7D, I felt the focal length might be too long, but didn't want the EF-S 60mm macro, when I expect to purchase a body with a full-frame sensor in a couple years. For anyone considering a 100mm Macro, I'm 6x6" tall, and was able to squeeze in an 8x11" sheet of paper on the floor, while standing with my 20D.
I evaluated the EF 100mm Macro and this lens side-by-side. The older 100mm Macro has a great reputation. From the specifications, you'll see that the new lens is slightly longer, and weighs more. The build quality is excellent and consistent with an L lens. Optics are precise. Color and Bokeh are outstanding, as review samples attest.
Auto focus and manual focus are smooth and deliberate. As with other Macros, the focus is precise, but moves slower than a standard telephoto lens of the same focal length. It takes approximately 2.5 seconds to focus from infinity to .3m. It takes approximately 3.0 seconds to focus from .3m to infinity, as the mechanism delays .5 seconds when autofocus is initiated.
The focusing limiter selector switch functions well, eliminating the time to focus, if you know your subject will be between .3m to .5m, or .5m to infinity. The ranges offer a good compromise between focusing element travel and practical subject distances. .3m to .5m represents a 180 degree turn of the focusing ring, and .5m to infinity represents about a 150 degree turn of the focusing ring.
I purchased this lens over the older 100mm Macro for the image stabilization. The image stabilization allows the hand held use of the lens under brighter lighting conditions. The image stabilization certainly behaves differently from other L lenses at 100mm. Telephoto IS may allow you to pan, this lens does not. Better shots will be obtained with IS on while tracking a moving subject; however, IS on this lens is no substitute for a telephoto with panning IS ability. The new IS technology does seem better suited for macro shots than earlier IS techonlogy. The subject seems to "stick" on this lens, as compared with images that seem to "float" with other IS lenses.
IS compensates for movement quite well, but I will shoot low-light subjects on a tripod. With IS on and and shooting at 2.8, the depth of field is extremely shallow. Any movement toward or away from the subject will result in an out-of-focus image. My 1.6 sensor certainly exacerbates the problem. If you need to obtain a more adequate depth of field, you must shoot at 8.0 or above, which will require longer shutter times under low lighting.
Outdoors, this lens will provide unique opportunities in allowing one to complete hand-held shots of bright-lit subjects, especially if you have a full-frame sensor.