Justin, after some forthright comments from me, challenged me to do a review of the 300mm f/2.8 II. I am not up to writing a review but I will say why I bought the lens and give a few examples of how it has performed for me. Perhaps this thread will encourage others to submit their thoughts and photos.
I drifted into bird photography after going on safari 3 years with a 7D and brand new 200mm f/4 L IS, then progressed to a 400mm f/5.6 L and then 100-400mm f/5.6 L. After a birding holiday in Brazil, I was introduced by a Belgian to the birdpix.nl site where I submitted photos. Most were rejected for being too soft, and I was astounded by the quality of the mainly Dutch amateurs with their big white primes, usually 500mm but with the occasional 600mm or 400mm f/2.8, plus teleconverters. I needed to upgrade my gear.
After much thought and studying all the websites I plumped for the 300mm f/2.8 II, which is not the favourite lens of bird photographers, for my personal following reasons. Firstly, I do not use a tripod but always do hand held as I like to wander around with my camera swinging from my hand or sit in a hide. The 300mm is far lighter than the big 400, 500 or 600mm and is very easily carried all day. It is heavier than the f/4 400 DO but that is the same price and has little better IQ than the f/5.6 400mm and only two stops of IS. Secondly, the Canon MTFs and the TDP tests showed that the lens was exceptionally well matched to the new 1.4x and 2x TCs. So, the 300mm with the 2xTC is an exceptionally light and effective 600mm lens, and f/5.6 isn't too slow.
I collected the lens from Jessops on a Saturday morning a year ago and immediately rushed out before AFMAing and took some bird shots. The following full size (reduced to 1200x800 for uploading) and 100% crop from the middle (800x880) was about the 10th photo I took and just blew my mind as I had got no where near this quality before. It is of a sedge warbler collecting insects for its nest - 300mm, f/2.8 Canon 7D. I'll follow these with examples with the 1.4x and 2xTC.
I hope these show that the 300mm f/2.8 II provides us with outstanding 300, 420 and 600mm primes.