If you're looking at this lens, you're ready to spend some serious dollars. Is it worth it? Short answer, yes, but I'll elaborate.
I've shot with a variety of Nikon and Canon lenses for about 25 years. The 300 f/2.8L IS is without a doubt the sharpest lens I've ever had, and it is unbelievably consistent across every aperture. With the 1.4X Canon extender - other its becoming an f/4 lens - I could not see any appreciable change in its performance. Unbelievable.
I got my introduction to image stabilization on the 70-200 f/2.8L IS, and it is even more useful on the 300. I handheld consistently crisp shots at 1/60 and sometimes at 1/30. With my old Nikon 300 mm, anything below 1/250 was out of the question and 1/500 was a good idea. The whirring of the IS is a bit louder on the 300 than the 70-200.
When shooting action in AI Servo mode, the autofocus was as fast as on my shorter prime lenses.
In some forums, people swear by third party long lenses that they say produce as good an image at less than half the cost. All I can say is that their experience probably reflects the limitations of their skills rather than the comparability of the lenses.
With the 1.4X extender and my 1D Mark IIN's 1.3X FOVCF, this becomes a 546 mm lens. On a 1.6X FOVCF camera it's a whopping 672 mm.
Here are some other Canon alternatives and why you might want to go with them instead of the 300 f/2.8L IS:
- Canon 300 f/4L IS - less than half the price; it is lighter and has a built-in lens hood that doesn't require assembly; you lose a stop, and while a very sharp lens, not in same class at the f/2.8. good choice if you don't have the money for the f/2.8 or want to spread it among other lenses.
- Canon 400 f/2.8L IS - a longer reach without losing a stop but the price and weight increase significantly; not really hand-holdable; lens of choice for outdoor sports action.
- Canon 500 f/4L IS - if you want to shoot birds, you'll need the extra reach. It's cheaper than the 400 2.8 but a couple thousand more than the 300. Sharpness is comparable to the 300.
- Zoom lenses (70-300mm f/4.5-5.6 DO IS; 100-400mm f4.5-5.6L IS) Cheaper, lighter and more flexible but a serious compromise on image quality vs. 300 f/2.8L IS.
Best argument against the 300 f/2.8L is that you won't shoot often enough at that focal length or you want to buy more lenses for the money.
I'm saving my pennies now to add the 500 f/4L IS for bird and other wildlife distance shooting. It will take a few years.