I had a really functional Canon kit that served me well--from 30,000 ft. I had all that I really needed in a system.
6D, 60D, 200 2.8L ii, 135L, 50A, 17-40L, 580exii
For sports, events, landscape and portraits I had all the tools I could ever need to take good pictures. But there were two things that bugged me, and it lead me to swap systems.
1. First and foremost, I would look at the files Sony shooters on this board were getting . . . and I couldn't stop looking. The detail and clarity they were producing was truly inspiring, and maybe I needed a little inspiration at that point. It took me awhile, but I became convinced I could go from my above kit to an A7R, a6000, FE 55 1.8, PZ16-50 and a lens that would give me 200mm f2.8 and be happier.
It took some trial and error with the 200mm solution, but I eventually got there when I accepted that the A77ii was going to be my telephoto sports shooting solution. I could sell my A77ii and Minolta 200 2.8 straight-up for a used 7Dii/EF200 2.8 combo but it would be splitting hairs IMO. But that is a different thread, probably.
The files I get from my Sony gear, by in large, are ones that I am happier with using my old Canon kit. My hit rate is also better, in spite of how slow the A7R can focus and how careful I have to be to use good form with 36MP behind every actuation. My A7R/FE55 combo is definitely coming out as an upgrade from my 6D/50A combo, in spite of it being an almost even trade-off financially.
2. Size mattered to me. I was bringing my 6D with me less and less because I had my two small children with me more and more. It wasn't that I had a problem fitting it in the diaper bag (read: lowepro 35L), just that it can be a challenge trying to navigate even a light body like the 6D when it has a 50A or 135L attached to it. A few whacks to the head to my toddlers when I had my camera slung around my shoulder or neck produced laser beams and gasps of pain. The a6000 is only a little bigger than a point-and-shoot with the 16-50 on it, so it is pretty harmless. The A7R with the FE55 attached isn't quite as small, but it is still small. I don't leave my camera at home anymore, and I don't worry about beating my littles around with it either.
So after about 10 months of shooting with Sony, I'm a believer. I feel my work is stronger than it has ever been, I can push the files really far in post when I need to, and the gear is more convenient for me to integrate into my life.
Sony A99ii, RX-100ii, Sonnar T* 135mm f1.8 ZA, Planar T* 50mm F1.4 ZA, 24mm f/2 SSM Distagon T*, Minolta HS 200 2.8 APO, Minolta 2xTC APO, HVL-F43M