I faced a big decision, my EOS-400D was getting old and its poor performance above 100 ISO prevented me from making a lot of images. It had to go, but what to replace it with? I could buy another Canon DSLR, and to get the functionality I wanted but that would mean something like a Canon 7D. If I bought a 7D I would no longer be happy with the performance of my lenses, so that would mean major expense on the camera and the lenses. I didn't want to spend that much money nor did I want to be carrying anything that heavy. There is a saying that the best camera is the one you have with you, and I wouldn't have a 7D with a big lens with me very much.
Another factor is that I also own a Canon S95 compact. The experience of using the S95 has been enlightening, I have never enjoyed using a camera as much and used within its limits it has produced some excellent results. I can make good prints up to 400mm wide from RAW files shot at ISO 80. It does take careful processing to achieve good big prints and care has to be taken not to accentuate noise.
So, I wanted a small light camera with good high ISO performance and interchangeable lenses. I also wanted as big a sensor as possible, given that the laws of physics mean that bigger pixels mean better high ISO performance and dynamic range.
In the end I went for a Sony NEX-6. I chose it because Sony have proved that they can make great sensors and have put one of those sensors in the NEX-6. The NEX range are also very small but solidly built.
I didn't buy a Canon EOS-M because its focus performance is poor and Canon don't yet have a credible story for the compact system camera format. It is notable that several long time Canon users in Storrington Camera Club are downsizing to CSCs and switching either to Panasonic or Sony. Nikon have a better CSC position than Canon, but they have gone for a small sensor design and although they have achieved remarkable results with it, it doesn't fill the same market position as Sony NEX or micro four thirds cameras.
So, I now have a NEX-6.