I did also return my 300mm 2.8 since that is just too much. I would prefer to wait until canon answers with a revised 100-400. Events and portraits still remain my main focus.If the 300 f/2.8 was too much, then I think the 1D X is probably going to be too much as well. The 1D features are really nice, but there are simple workarounds to most of them that can be done with the 5DIII. I think the 1D X is really for those needing one or more of the following: combat-ready build quality, 12 FPS (are you missing a lot of fast moving shots?), or are someone who uses big whites a lot and and would benefit from the higher votage drive for faster AF, or for someone who has pretty much all the other lenses and gear they need. If wildlife or fast motion things like sports aren't a priority, lenses and other gear are a better investment. The 5DIII is an amazing camera and the 1D X is better, but (other than the frame rate) not mind-blowingly better.
Those are good points. I just want to clarify that the 300mm felt large and was so expensive for what it was. I felt if I was going that route I would save for the 400 with extenders for next season. Lately i have been doing some indoor water polo at a high school and wanted better files to work with in the iso 3200-8000 range. Also when I am chasing the kids I felt the multiple mn buttuns that can be configured on the fly sounded beneficial such as being in one shot and being able to prog my dof to aiservo and the other the all focus points if the kids/animal just moved to quick to react.
Keep in mind, the weight of 400mm f2.8 IS II is heavier, handheld becomes more challenge.
I bought the 300mm for indoor swimming, but like you said, it wasn't enough reach. In my case, it wasn't enough for close-up shots. I took a bite on 400mm f2.8 IS II and that was a right decision. With two bodies, it fits well with 70-200mm f2.8 IS II combo.
If you ever decide on 400mm f2.8 IS II, be prepare for decent tripod and gimbal head. I plan to add decent monopod to use with my Wimberley II in near future.
Just in case, this thread has great feedbacks from knowledgeable members: http://www.canonrumors.com/forum/index.php?topic=20609.0