I have owned it for years and it is one of those lenses I never thought much about. I bought it because I needed a relatively long lens that I could travel with easily by air and take on hikes. Years after its purchase I begin to realize how versatile this lens is, while going through images that spanned a number of years. This is definitely a lens I'd buy again. Let’s take a look at what makes this lens so versatile:
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1. Quality: Its optical and build quality are both very high. Although its optical performance is not as stellar as Canon's 300 mm f/2.8 IS L lens, it weighs and costs about one third as much, and maintains respectable image quality throughout its aperture range.
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2. Portability: As mentioned above, the lens is relatively small and light, making it quite portable. That means you are likely to have the lens with you when it is needed. Enhancing its portability, the effective, retractable, and very sturdy sun shade locks solidly into place and takes essentially zero space inside a camera bag.
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3. Hand Holding: Being relatively small and light makes this lens easy to hand hold when mounted on a camera body. At just 2.6 pounds with the tripod collar attached, it will not fatigue your muscles to the point of quivering for quite a while. Image stabilization takes this a step further, allowing hand held photography in situations where it would normally be impossible.
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4. Close Focusing: This is where the 300 mm f/4.0 lens gets a good deal of its versatility. The lens focuses as close as 4.9 feet. Compare this to the lens's faster f/2.8 cousin whose minimum focusing distance is 8.2 feet. At 300 mm, 4.9 feet gives a field of view of just 3.8 x 5.6 inches and a magnification of close to 1/4 life size on a 24 x 36 mm frame. Add a 2x converter the field of view becomes just 1.4 x 2.1 inches with a magnification of 0.7 life size. It is easy to see the lens can double as a decent macro lens that maintains a significant working distance of 4.9 feet. This distance is welcome when subjects are timid and may flee if you get too close. Also, keep in mind that the dimensions I have mentioned here are based on a full frame camera. Cameras with a digital "cropping factor" will have even smaller fields of view and higher effective magnifications at the same working distance. This is not to say the 300 mm f/4.0 lens can take the place of a lens like Canon's 180mm f/3.5 macro lens, but the extra working distance often makes the 300mm lens more desirable. In some cases this is not because you will scare off the subject, but because you cannot physically get any closer to the subject due to obstacles. It is also not always physically or financially possible to have every specialty lens available wherever it might be useful. The macro capabilities of this lens can fill what might otherwise be a gap in a set of equipment.

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5. Speed: The f/4.0 maximum aperture allows autofocus functions to work on all Canon cameras when using a 1.4x converter. Autofocus capabilities on Canon’s professional camera models are maintained even with a 2x converter.
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I recommend the lens based on the way it has performed over many years of use, so I will not go into specifications and MTF charts here. You can see all of that on the Canon web site. 

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Happy shooting!

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General Notes and References:

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1. Canon's 300 mm f/2.8 IS L lens is optically one of the best performing lens in the world. It is in a different class than the 300mm f/4 IS L lens, but general comparisons are made here because these lenses have the same focal length and manufacturer. These comparisons also drive home the point that in a number of circumstances the f/4 lens will let you bring home pictures that the f/2.8 lens will leave you without.

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2. The versatility of the 300 mm f/4.0 IS L lens is greatly enhanced by Canon's 1.4x and 2x teleconverters. Although these slightly degrade image quality, publishable results can be obtained even at maximum aperture with a 2x converter. The slight quality loss from using a teleconverter is insignificant compared with not getting the photo. The worst degradation occurs in the corners of a full frame (24x36mm) capture at maximum aperture using the 2x converter. This degradation is significantly reduced when using digital cameras with smaller sensors because they capture only the center portion of the image.

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3. Choosing a lens depends on many factors, including what lenses you already own or plan to own, what you plan to photograph, and how you do most of your photography. If you own or plan to own something like a 70-200 mm zoom lens, the 300mm f/4 may be a perfect addition. If not, Canon’s 100-400 mm f/4.5-5.6 IS L zoom lens might be a better choice. If you do lots of hiking and need a versatile and compact system, teaming the 100-400 mm zoom with a wider angle zoom can produce a two lens system capable of taking on a wide variety of subjects. As is the case with other zoom lenses, I would not recommend the regular use of teleconverters because image quality often suffers too much. But, with a maximum focal length nearly equivalent to that of the 300 mm lens with a 1.4x teleconverter attached (420 mm), the 100-400 mm zoom lens offers loads of convenience. On all but Canon's professional cameras, using a 1.4x teleconverter with the 100 - 400 mm zoom lens will require manual focusing. Using a 2x teleconverter with this lens requires manual focusing on all cameras and generally produces unacceptable results. Image quality improves significantly when stopping down the lens two or three stops, but that is sometimes either impossible or undesirable. The 300 mm f/4.0 lens can produce acceptable quality and still allow autofocus on Canon's professional cameras with a 2x converter attached. This gives some additional reach when needed at the cost of zoom capabilities. We own both of these lenses and both perform well. Unfortunately, there is no single lens that can do everything.