I went from a 5dmkii (and I still have a 40D) to an X-T10, and I did not regret it at all. The X-T10 is so much fun to shoot with, the image quality is excellent, and things that I didn't even think of beforehand were far superior than the 40D.. like accuracy of auto white balance etc. With the 40D, unless I did a custom white balance or shot a grey card in every scene, I inevitably spent quite a bit of time correcting the colours in Lightroom afterwards. With the X-T10, I only felt like I had to do this once per about 150 shots or so.
Another great thing for me from moving to mirrorless from dslr... no more front/back focussing problems! I had to micro adjust the focus on some of my canon lenses to almost their maximum settings to get them to focus on target. You won't find this problem with the Fuji cameras
My only two negatives of the X-T10 were the physical size, and the viewfinder. Now I have small hands, and one of the main reasons I went mirrorless was the smaller, more convenient form factor, but even taking this into account I found that I would be frequently pressing buttons on the back of the camera with my palm with got a bit frustrating. I ended up disabling the function of a couple of the buttons in the end so this wasn't a problem. Although the optional metal grip that Fuji sells for it helped a lot with this, and I would 100% recommend it (although is a rip off at about £80).
The viewfinder is amazing in terms of resolution, refresh rate and general quality... but the eye cup is not very deep, and in brighter light situations the image in the EVF can appear washed out due to the light seeping in there between your eye and the 'cup'. Not sure if there are other eyecup options out there for the X-T10, but might be worth checking out.
In summary, moving from a 40D to X-T10 is a no brainer move as far as I am concerned. You will really enjoy the experience of this fantastic little (but very capable) camera compared to your ageing canon beast. However If I had to do it again I would probably have gone for an X-T1, for the slightly more comfortable grip size and better EVF/eye cup.
