I became truly serious with photography while using a 40D, upgrading from a 400D/XTi. (This upgrade was for handling qualities and ergonomics, as the sensor was the same.) The 40D was replaced by the 50D, in the Canon line-up, though I skipped the 50D, wl3cted to leap two generations, rather than one. Then, when I noticed that the 60D was seemingly a down-grade in some ways, bought a 7D, which I believed was the logical continuation of the "semi-professional" category. (The 60D had controls more like the consumer-level Canons, among other differences, and skimpier gripping areas, that annoyed me.) The 7D Mark II seems to be a further divergence from the 60D-70D-80D series, a true professional's APS-C DSLR, also suited to the keen enthusiast.
I added a pre-owned 5D, while I was using my pair of 7D cameras. A friend, who had a 30D, handled my 5D, and said it was built like his 30D. I did some internet research, and reached the conclusion that the 5D, due to lack of weather-sealing, should be considered consumer-level in build quality, by today's standards. The 5D Mark II represented an elevation to a true semi-professional camera body at the full-35mm-frame level. (Of course, plenty of professionals have used the original 5D, with satisfaction.)
The 1D-series, with its intermediate-sized APS-H sensor, seems to be no more, with the advent of the 1D X series, which has a full-35mm-frame sensor. The 1D X signified a merging of the older 1D-series and 1Ds-series, into one full-35mm-frame camera. Shooters who have held onto their 1Ds Mark III cameras, for its higher resolution, should probably look to the 5Ds-series in their quest for more MP.
It remains to be seen whether the 5D-series splits into the 5Ds-series, for high-resolution, plus another 5D-something for general-purpose shooting. (5D X, and 5D Mark IV, are rumored to be the names for the new camera. Time will tell.)
The 6D is, physically, something like a 60D with a full-35mm-frame sensor, and no articulating LCD. Its central AF point is very capable, with the outer AF points, and overall AF system, less-capable than pro/semi-pro cameras. There is no way to know where the 6D-series will go; it could be a "one-off," or perhaps it will become the basis for Canon's next semi-pro camera.
I cannot really discuss the consumer-level Canon DSLRs, having paid little attention since upgrading from the 400D/XTi. This should not be interpreted as snobbishness on my part.
I did buy an EOS M3, for the relatively few occasions I want a smaller camera body.
This is not intended to be an unbiased narrative; much of this is, indeed, my opinion.
I use 7D Mark II and 5Ds R cameras.
Canon 5Ds R/7D2/7D/5D/40D/1D2N/M3; Nikon F6/D3s/D700/FM3A/1Dx/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 24-70/2.8G.