I switched from Canon APS-C (which I have been shooting for 6+ years) to Sony APS-C mirrorless back in 2013.
The greatness of Canon system is in lenses.
The mirrorless systems have good lenses, but the choice is poorer, especially in the high quality mid price range fast zooms (although CaNikon imho mainly benefit here from the 3rd party lenses from Tamron and Sigma, Canon 17-55 is not mid price). Even in the entry level lenses, Canon has some real gems which offer a lot for relatively little money (55-250, 10-18, 85/1.8).
The Sony APS-C sensors are better in some respects (DR, high ISO noise - but especially DR) but to me, it's not critical. And it took me a while to get a lens line up I can live with, although I still wish I could just find the direct replacement for my Canon lenses (I had Sigma 17-50/2.8, Tamron 70-300 VC and Canon EF85/1.8, had I stuck with Canon I'd add 10-18).
The biggest difference is the size. I had to carry a backpack to bring my Rebel and 3 lenses along, covering 17-300 range. Heck, I had to carry a backpack just for T3i + 17-50.
With Sony, a really small (I think it's 11 x 13") and thin messenger bag that my wife used to carry her iPad around fits the body and 3-4 lenses in the 12-105 range (including a fast portrait prime), and I could easily add a 55-210 (which I simply have no need for) with room to spare. And for the casual shooting, I can mount a small prime and it would fit in my jacket pocket. Also, a big advantage for casual portrait shooting is the built in flash that tilts so it can be bounced off the ceiling. And using MF legacy lenses is a breeze, fun, and very cheap.
About the only thing I miss from my Canon days is Canon skin tones. I really liked them, and now have to tweak my RAWs to get close.
Canon can still catch up. Sony is now the king of the mirrorless market, and more or less own the mirrorless FF market, they have some very nice glass for FF, but I find their lens offering for the APS-C a bit scarce and often overpriced (the $800 10-18 is a great lens but too pricey IMHO). And so far APS-C cameras far outsell the FF cameras (this may change). If Canon came up with a decent APS-C mirrorless (EVF, tilting LCD, IBIS, a bounceable built-in flash, fast AF and a fast MF assist like focus peaking are a must for me), expanded their current EF-M line up to include a fast zoom and a scaled-down version of 85/1.8, and offered an adapter that would provide the full AF speed with EF glass (Sony can do it with A-glass on E-bodies, so why not Canon ?), they could grab a huge chunk of the market. OTOH, in 2015 Sony is rumored to come up with a Nex-7 successor which would have fast AF of A6000 and 5-axis IBIS, which would make a killer APS-C camera (there's plenty of good and decently priced Sony and Minolta A-mount glass out there and it provides a full AF speed with their adapter but no stabilization with current bodies). There's also rumor of Nikon working on FF mirrorless, they already have a very decent m43 offering (not too good in low light but superfast AF), so they are only one step from coming up with a competitive APS-C mirrorless as well. Canon has better start moving, and fast.