Recently, I rented 300/2.8 IS lens for a commercial photo shoot. This is the kind of image it produces at f4. f2.8 aperture comes into play even so, making focusing much more certain and accurate.
It also works great for portrait and figure photography and, with 1.4x teleconverter, for wildlife. I considered an Tokina/Sigma/Tamron route but the lack of image stabilization limits their usefulness. I used 300/4 IS enough to regard it as a stop-gap measure due to lower image quality and smaller aperture. I've got to the point where I can easily see the difference between a merely decent lens (like Tamron 28-75/2.8) and a good lens (Canon 50/1.4) and a superb lens (Canon 135/2). For the readers of my site, the payoff would be in getting to see images which would not be possible without this particular lens. In Nashville, I can't rent is as needed, so that route is out.
My wish list at B&H The lens is just under $4K right now. I managed to save a little over 1.5K towards it.