I really love the ergonomics of ALL the Canon bodies. But like anything, there is a learning curve. I read a lot of comments about weight and size and such, but to me, none of that has ever bothered me. Loved the 7D. Loved the 6D (although wish it had a bit more to it, but it's a base model, so ...OK) and I love the 5D3. I've seen a few remarks where people claim to prefer the "size" of the 6D over the 5 series...but honestly I don't get it. The difference between the two seems pretty insignificant to me. Now going from a Rebel t2i to a 7D .... ok, THAT was a wake-up call. But once you're in the pro bodies, I can't tell much difference except for the 1DX which I also love shooting. I shot a 1DX non-stop for 3.5 hours at a time, 4 times over one weekend for 4 huge dance recitals with the 70-200 IS 2 strapped to it. My upper body was whipped but shooting literally 8000 frames for that long, I would have been just as whipped with a 7D, a 5D, or a 6D. Plus...the 1DX just looks like a bad ass. I'll be first in line for a new 1DX2 this year. Weight and size be damned. Get the best camera you NEED for your type of work. If it's too heavy, I'd respectfully suggest the gym and develop good PHYSICAL technique and body posture for long shoots. Straight back, use your legs and core to stabilize your center of gravity, relax your upper body completely. I know it sounds a bit silly, but I practice and teach martial arts too, so that sort of talk is gospel for us. It translates extremely well into fast paced sports photography.
It's not too heavy, it's too big. I regularly shoot with a 20D+70-200/2.8 and 5D+24-105L, both with 580EX's attached, simultaneously for as long as 12 straight hours while also carrying 3 other lenses and accessories in a waist pouch. The problem with the 1D cameras for me is that the grip is too thick. Well, that and I can't remove the useless (for me) portrait grip. It's just in the way.
I realize this is mostly because I have little hands. But the 5D series fits perfectly.