The Canon EF 100mm f/2 USM Lens - meet the Canon EF 85mm f/1.8 USM Lens' twin sibling. Put these two lenses side by side and you will need to read the numbers to tell them apart. That is a good thing.
Putting numbers to the size and weight ... The 100 f/2 weighs 1.0 lb (460g), measures 3.0" x 2.9" (75.0g x 73.5g)(DxL) and has a relatively small 58mm filter diameter. This is relatively small and light - This is an easy-to-take-with-you lens.
The Canon EF 100mm f/2 USM Lens also shares the Canon EF 85mm f/1.8 USM Lens' fast and quiet Ring USM (Ultrasonic Motor) focusing. Tracking action sports is a task well-suited for the 100mm f/2 - AI Servo focus accuracy is excellent.
Contrast and saturation are very good - comparable with the sibling. The optional ET-65III Lens Hood is a wise addition to this lens - to retain the contrast and saturation with bright lights in/near the frame. Some CA is apparent at f/2. Wide open vignetting from the 100 f/2 will be apparent but not bad for full-frame DSLR users - similar to or slightly better than the Canon EF 85mm f/1.8 USM Lens' The 100 f/2 is one of the better lenses available in regards to distortion – expect only a negligible amount.
The 100 f/2 also shares the Canon EF 85 f/1.8 Lens' wide 8-blade aperture. Low light action sports is a specific task the 100 f/2 is well suited for. This lens works well for indoor events. The less-demanding daytime outdoor sports should not be a problem for the 100 f/2, but many prefer using a good zoom lens (such as the Canon EF 70-200 f/4 L USM Lens or the Canon EF 70-200 f/2.8 L USM Lens) for these events. The zoom range enables better framing of your shots.
The wide aperture on the Canon EF 100mm f/2 USM Lens is able to create a diffuse background blur when called upon. Read my article on backgrounds for more info.
The 100mm focal length combined with the wide aperture makes this a very good portrait lens. Selecting the length of a portrait lens is a combination of functionality and personal preference. 100mm falls in the middle of my 85mm-135mm preference for a portrait lens. Head and tight head/shoulder shots are what I find most comfortable with this lens when mounted on a 1.3x or 1.6x FOVCF body. That said, I would choose the Canon EF 85 f1.8 Lens if I were to have one similar-priced portrait lens on a 1.6x crop DSLR. The 100 f/2 feels a bit tight indoors on these cameras.
The Canon EF 100mm f/2 USM Lens is a very sharp lens. I found the 100 f/2 to be sharp at f/2 and only improves slightly when stopped down. Well, all lenses become less sharp somewhere around and beyond f/16 because of diffraction. Corner sharpness is good as well.
The 100 f/2's maximum magnification is a meager .14x at 3' (.9m). Extension tubes make a big difference in this figure - .27x and .42x for 12mm and 25mm tubes respectively.
Positioned above from left to right in their fully retracted positions are the following lenses:
Canon EF 50mm f/1.4 USM Lens
Canon EF 85mm f/1.8 USM Lens
Canon EF 100mm f/2.0 USM Lens
Canon EF 135mm f/2.0 L USM Lens
Canon EF 24-70mm f/2.8 L USM Lens
The same lenses are shown below in their fully extended states with their lens hoods in place.
Here are my recommendations ...
If you want a prime (non-zoom) lens to use for portraits and do not require a 100mm focal length, I recommend you get the Canon EF 85 f1.8 Lens. Of course, if your budget allows, get the Canon EF 85mm f/1.2 L II USM Lens or the Canon EF 70-200 f/2.8 L IS II USM Lens. The Canon EF 135mm f/2.0 L USM Lens is excellent too, but it might get a bit long for general indoor portraits on a 1.3x or 1.6x Digital SLR.
If you need a 100mm lens and can get by with a minimum aperture of f/2.8, consider the slightly more expensive Canon EF 100mm f/2.8 USM Macro Lens or Canon EF 100mm f/2.8 L IS USM Macro Lens. You get a very sharp lens with an additional capability - 1:1 macro photography. I do prefer the handling of the 100 f/2 over the 100 macro lenses.
If you need a fast-focusing low-light action sports type of lens, the Canon EF 100mm f/2 USM Lens is similar to the Canon EF 85 f1.8 Lens - and the Canon EF 135mm f/2.0 L USM Lens is a slightly better choice (in my opinion of course).
The Canon EF 100mm f/2 USM Lens is a very good lens at a very good price.