Conclusion: I bought this as an upgrade for my current f4.5 EF 300. I wanted betterr magnification than the old 300 (or my 200-400 zoom). Ideally I would like a 600mm f2.8 that was as light as this lens. I'm a bird photographer and am always reaching for more distance without having to lug huge lenses around. This lens with a 2x extender is the perfect compromise for me. The only downside is that now my lens is better than my camera body and I want to upgrade that too.
Pros: Fast auto-focus, Superior build quality, Super-sharp images, lighter than the older model
Conclusion: Since this is my fourth Canon 300mm f/2.8 lens, I'm very aware of its evolution over the last 30 years. Each model has come with improvements both optically, and mechanically, but secrectly I've always longed for the size/weight of my original, the new FD 300mm f/2.8L. The first two EF's manufactured were both great lenses, but their weight/size were just a tad bulky for consistant hand holding, even having added IS on the second version.
Conclusion: Ridiculous was an adjective used by someone I know, when other superlatives were too weak in their description. There are a couple of lenses in my arsenal that are considered by many to be reference standards. But this one even leaves those behind and has no peer - in many different aspects. The image quality will blow you away, words are useless here, the pictures you get will astound you. Razor sharp across the entire(!) frame and bokeh that is crazy delicious.
Pros: Fast auto-focus, Superior build quality, Super-sharp images
Conclusion: I am an inexpert bird photographer, and have graduated from a 70-200 L IS with a 1,4xTC, then a 400m L f/5.6 and a 100-400mm L. They are all fine lenses, and I love my 100-400mm for travel as it is light enough and is small when the barrel is compacted. But, I wanted the longer reach of a 600mm which could be hand held as I want to wander around holding camera and lens without a tripod, and also an upgrade in image quality.
Pros: Fast auto-focus, Superior build quality, Super-sharp images, perfect with tcs, not too heavy, with tc a light supertelephoto