I went from 35 years with Nikon up to a D800 to Fuji. Now shooting an XE2 and X-Pro1 with a pretty full complement of Fuji glass. Never looked back.
Pro's: Weight, size, IQ, metering up to Nikon standards, Auto WB nails it every time (with Fuji's JPEG's that's important), AFS comparable to the D800, tracking not. A joy to use. Excellent build quality, rugged. Excellent fast lens line that, once you start assembling a kit, makes a lot of sense. All the lenses are high quality. You buy speed/size vs. price.
Cons: At the margin of DR, you give up a bit. I've only seen this in one important shot, a sundown in a cloudy sky off the U.K. coast. The person I was shooting with has my old D800. His view is they were a wash. Resolution is obviously different. But I don't print large with most of my shots going to the web for viewing. Somewhat higher priced than Sony AFC or m43. Fewer bells and whistle "features" than the competition. No IBIS (as an older shooter I want this -- the reason I've stalled upgrades within Fuji). The longer lenses have VR.
I have no doubt there are better cameras out there (XT2, XP2, A7's). But There's a joy I get shooting these cameras that I haven't been able to match when I trial others. Fuji just get's the basics right. Unless you're shooting kids or sports where you want current gen AF.
I started off by using my Nikon AiS glass. Works fine. If one has an inventory, may as well use it. I would not mount Nikon AF glass to these bodies. Too big, unbalanced.
One can pick up older Fuji bodies for a song these days. Probably find a refurb for the fraction of the price of a new model. My XE2 is a refurb, my XP1 used. I put my money in glass instead.