I picked up a used 1D mkIV, my first 1D series camera with built in grip. My main camera is 5D series (first mkII and now mkIII), so I am used to a fairly heavy body. I have never had an issue (though I often use a tripod).

I used the 1D mkIV with my 70-200 f2.8 handheld for a couple hours at a private photo shoot at a wild cat center. (I had my tripod but ended up not using it). Ever since that trip almost three weeks ago my right elbow has been hurting when I use it for anything (even brushing teeth). Went to urgent care today and I have tendonitis (aka tennis elbow), presumably as a result of the shoot with the heavy gear.

So if anyone is considering a 1D series, make sure you are careful and use a strap or tripod whenever possible. (I stupidly did not get a strap so I carried it by hand the entire time).