OP here... Just wanted to add that I would have been fine with my 5D II, heck probably even with my original 40D. I'm still using the center focus point for 80% of my shots even when I have 61 available. With that said, 3 things that I felt was worth the upgrade based on my experience with the 5D II.
1) Low light focus - Even when I still use the center single AF point for the majority of my shots, I am able to focus in low light. I have lost so many photos with my 5D II because I couldn't focus in low ambient light.
2) 6 FPS - My first DSLR camera was the 40D. I learned everything there is to know about DSLR photography on that camera and I found the frame rate to my liking. When I upgraded to the 5D II, 3.9fps didn't allow me to take hand-held HDR or capture fast moving objects for the times that I needed to. With the 5D III, I now have the speed I need again. I considered getting the 1Dx, but I didn't need 11fps and would rather have more resolution and a compact body for portability.
3) Better ISO performance - I just came back from Hong Kong & Bali and not once did I take out my tripod. I don't even know why I brought it with me to only add weight and space to my luggage. I was able to take clean photos, handheld at night, with little noise.
Was it worth the upgrade or the extra $1000 over the 5D II? I don't know, to me yes but it may not be worth $1000 more or a whole new $3500 to others. I was able to sell my 5D II for $1,700 which I think is pretty good for an electronic gadget that has been used for 4 years. That's $900 of depreciation in 4 years of ownership! If you compare the 5DIII to the D800, it is not worth the extra cost IMO. Both good cameras, but I'm too invested in Canon lenses and will never see myself jumping ship.
Yes it is an expensive camera and there will be haters. There are people who saved up for the 5D II to only have the 5D III be introduced a short time later. Buyers remorse and envy of shiny new things will generate a lot of negativity to justify their purchase. I think people who end up foul mouthing the 5D III or newer versions of their camera are too focused on "specs" / "features" and not so much in perfecting their photography skills. I have never been so serious with any hobby in my life until I picked up my first DSLR. I try to hone my skills by taking classes, workshops, attending meetups to learn from others, and fortunately have a good day job to fund my equipment. I'd like to think that I've improved over time, but I know a 17 year old with an old Nikon entry-level DSLR who can still kick my ass in composition and general artistic value.
It's all about the photographer behind the camera. The camera just allows you to expand your creativity and provide performance to meet your demand. The best thing to do as a photographer is to not read the forums and just get outside and take photos.
If I had to say anything negative about the 5D III is the slight underexposure which many have complained about. I do notice that in some lighting conditions where the 5D II would be spot on, the 5D III tends to underexpose them slightly. I don't think it's a problem, but simply something owners have to get adjusted to as it is a whole new metering system. I just need to review the histogram closer for shots that don't look right on the LCD.