I've been testing out the Canon 300mm F/4L IS lens this weekend and so far the results have been amazing. It managed to handle low-light situations just fine at the NCAA Track and Field Regionals, and compared favorably to the 300 F/2.8L IS (which costs and weighs about three times as much). Below are two images of the same athlete photographed two years apart, one with the 300 F/4L and one with the 300 F/2.8L. See if you can guess which is which.
The top image was shot with the 300 F/2.8 and the bottom one with the 300 F/4. I actually prefer the bottom one, but either way, both results are perfectly usable.
It also turns out that the 300 F/4 is an EXCELLENT macro lens! I spent the morning shooting wildflowers with it and I was blown away by the results. The lens focuses down to a mere 4ft (amazing for a 300mm lens!) and the results are also very good. The lens is incredibly sharp. I was also impressed with the shallow depth of field with a smooth and pleasing background blur. This is an excellent nature lens.
I'm very happy with what I've seen from the 300mm F/4L IS lens that I rented for the weekend and I think I will be extremely happy adding one to my kit in the near future.
Check out more shots at 500px.com/cnstoll.
This review also helped inform my decision.