I've been using Canon digital cameras since about 2005 when I purchased a second-hand Canon EOS 300D to use alongside my Canon EOS 1N and EOS 5 film cameras. (I also used Pentax 67 medium format cameras at the time.)
The 6.3mp 300D changed my photography forever. I could see immediately whether I had got the shot or not and I loved it! At the time, I said digital would never be good enough to replace my medium format gear. Only a couple of years ago a client wanted to use one of my images for use in his dentist surgery, on the ceiling, 6 feet x 4 feet. I tried to persuade him to use a later image taken on a camera with more mega-pixels but he liked the early shot. Once printed, I was amazed at just how good that camera was as long as the exposure was bang on.
The next camera to come my way was an EOS 20D. The images were exactly the same as the 300D but build quality was a little better and felt good in the hand especially with the vertical camera grip attached.
My next purchase was my first brand new digital camera, the newly released 8.5mp EOS 350D. I had been bitten by the chasing mega-pixels hype! Unfortunately this camera did not live up to the hype at all, the images were never as good as the previous cameras despite the pixel count and were soft and noisy. Fortunately, Canon had released the full frame 12mp EOS 5 at a good price and as soon as I could, I bought one and an L series 24-105 lens. The difference in image quality was staggering and spelled the end of my film shooting days on medium format. ( The 35mm cameras had been retired with the 20D era I think.) I'm not sure why but the 5D suffered from dust on the sensor more than any other camera I've owned. I got very good at cleaning it and shot almost 100,000 images on it with no problems at all.
After a good aerial photography season, I had a rush of blood to the head and bought Canon's flagship EOS 1DS mkiii. This 21mp monster was brilliant to use and gave excellent image quality as long as the ISO was below 1600, above that and things could get a little noisy. A slight disappointment with the 1DS mkiii happened after my first big aerial shoot when many of the images weren't pin sharp. I contacted Canon and was told it wasn't one of the serial numbers that had a known problem but would look at it anyway. To my amazement, it was collected from home, adjusted and returned to me in just three days. From that day on it was flawless and I loved using it and the way it felt in your hand. The only thing I didn't like was the weight if travelling abroad.
After using the 1DS mkiii for about 2 years Canon released the 5D mkii to great acclaim from everyone who'd used it. Around this time I was shooting a lot of airshows and sports and I decided I'd be better off with an EOS 5D mkii for aerial photos and landscape work and a 7D for airshow and sports stuff.
In terms of image quality the 5D mkii was certainly better than the 1DS mkiii and also the 7D but the focus was was not quite as fast as either especially the 7D which is lightning fast and very accurate. The 7D's build quality felt much better than the 5d mkii and it felt almost like you were holding a baby 1 series camera.
For years I had been trying with various degrees of success to geotag my images, something Nikon owners had been able to do for years. I'd used Nikon D200, D300 and D3 cameras with external gps units very successfully but unfortunately I simply don't like using Nikons! When Canon announced they would be introducing the 20mp full frame EOS 6D with built in gps and wifi my dreams were answered, as long as it produced great images. After much research and hours spent reading reviews and the obvious comparisons with the much more expensive 5D mkiii I bought one. The image quality is the best I have ever seen from any DSLR and is virtually noise free right up to ISO 3200 and perfectly useable up to 12800! The 6D feels great in the hand even though it is a small body. I usually have the vertical grip attached and have yet to need to change batteries during a busy days shooting.
The only thing that the 5D mkiii does better, is that it has similar focus speeds to the 7D but at around £800 more than the 6D plus another £200 for an external gps I think for me the 6D wins hands down!
I'd read people complaining that the 6D only had 11 focus points, so what? Most of us only use one anyway. Occasionally I'll use the outer ones but never have I thought 'I wish I had 63 focus points!' In the few months I've owned the 6D I've shot loads of aerial photos, 2 weddings, 2 overseas trips and loads of other bits and bobs and I'm still so excited about using it.