I can only recommend this lens. I use it on a T3i and like it for its IQ, the compact size (retracted) and all the usual L stuff like build quality, AF etc.
The reversed zoom and focus rings are not an issue for me, I just hold the lens in the front and it does not feel unnatural at all, the barrel is not that long. But I might be a little more used to it because my 15-85 is the same way.
I chose it over one of the 70-200 because of the extra reach and the compact size when retracted - it fits vertically in my ThinkTank Urban Disguise with camera and lens hood attached. IQ is supposed to be better than 70-200 + 1.4 converter, according to TDP.
If you want to shoot wildlife/birds often, you might even want to consider the 100-400. 300mm is not very long for that, even on a crop camera. As others have reported, the 70-300L works well with the Kenko TC.