Pros: This is an impressive lens which was released 1997 and is still in production because it is inexpensive, lightweight and only one stop slower than the f2.8 Canon lenses lenses at ~1/5th the cost

Cons: Photos produced with the 1.4 MK III extender look terrible in large formats/prints but acceptable at smaller formats (about the size of a sheet of paper). Just buy the 400mm instead of cheaping out on an extender and your photos will be professional quality.

Other Thoughts: Consider the following: The 300mm f4 is ~$1400 and weighs 1.2kg, the 300mm f2.8 IS II costs $6,600 and weighs 2.4kg. With the money saved one can buy a few other lenses or a good full frame body while ALSO saving the back aches associated with lugging 2.4kg of f2.8 glass around on all day adventures.

The choice seemed quite simple to me and I am sure that you will not be disappointed. Just dont throw a 1.4 mk III extender of this lens, the results are not professional grade.

Pros: Easily handholdable; Image Stabilization (IS) is excellent; Image Quality is fantastic; Sized perfectly for a shoulder bag with or w/o camera mounted; Even with IS and Auto-Focus in-use, battery life on the Canon 40D w/battery grip is excellent and worth at least 1000 shutter actuations; works well with the canon 1.4TC mk II.

Cons: As other have reported, IS mechanics are audible. In my experience, that has never negatively impacted shooting subjects in nature.

Other Thoughts: Shooting with a xxD body (40D) with a battery grip and this lens is very reasonable for average sized male hands. A person with very small hands or a physical disability might find this combination unwieldy.

Pros: Excellent build quality. Sharp wide open - tack sharp when stopped down to F/5.6. Fast/accurate AF. Takes the Canon 1.4X extender reasonably well. Convenient built-in hood.

Cons: The IS system is a bit clunky/noisy compared to the latest versions in newer lens designs but easily delivers the claimed 2-stop assist. With good technique, I'm getting sharp handheld shots on a crop sensor camera at shutter speeds of 1/60 - sometimes a bit lower.

Other Thoughts: IQ and build quality of this lens is comparable to my 200/2.8L and 400/5.6L (which is to say - excellent!). Initial acquisition and AF speed of the bare lens is excellent - just a bit slower than the 400 (which is VERY fast). AF speed (moreso initial acquisition speed than tracking) and IQ are noticeably effected with the addition of the Canon 1.4X extender, but performance is still good. Given the features and performance it's a great bargain at this price.

Pros: Low weight (vs. 300 f/2.8)
Fabulous IS system
Optics up to par with the 2.8 lenses
Fast AF

Cons: Mediocre performance with a 1.4x teleconverter (Tamron)

Other Thoughts: Though the images from the Nikon 300 f/4 have a little bit more bite, the IS on the Canon makes it a winner. Hand-held shots at 1/60~1/125 are completely acceptable with this lens. Try that on a non-IS 300!