Just a couple days I received my new 6D - and what a fine camera! I was just getting started testing the new 24-105 when I noticed a prominent spot on every photo. It was a sizable dust speck on the sensor. The sensor cleaning function didn't help. And in the viewfinder there were several pieces of visible dust (more of an annoyance - they don't show up on photos). So I sent it back to B&H this morning and I hope the next one is better.
I understand that some would probably have kept it - but being a poor photography enthusiast and having saved 3+ years I didn't see why I shouldn't exchange it. My 6 year old Rebel XTi has a few specks in the viewfinder but nothing on the sensor.
So I patiently wait......
My first 5D mk3 had dust spots all over the sensor. I tried blowing them out, a few went but the majority stayed. I took it and got it professionally cleaned and one dust spot was still there, showing up at f11. I took it back had it cleaned again, still there! Turned out the dust was between the filter and sensor. The 2nd one was perfect!