NEX is very different to using a DSLR but it's a very 'userfriendly' camera and I'm sure you could get the hang of it very quickly. Personally I MUCH prefer it to a DSLR and have got vastly better images from it.
The NEX does have very extensive menus and, to be honest, they are not too well arranged. However, the big redeeming feature of NEX is that the UI is cutomisable to the individual users liking. The camera is effectively managed with just the three buttons on the back of the camera,..the centre button surrounded by the selection wheel,....I have this cutomised for 'meter mode', 'iso' and 'drive'. In each the wheel can quickly select what I want. The bottom button I have set for 'manual focus assist',.....only applicable when using my Canon FD lenses with adaptor. 'MF assist' instantly gives both 7x and 14x 'blow-up' of the subject and this can be easily shifted around the image to check focus. When using manual focussing lenses on a tripod the 'feel' is very like using a miniature technical camera except that the NEX screen shows the subject the right way up and you don't need a black cloth over your head!. Finally, this still leaves the top button to reach 'menu' should I need to do so which is rare in practice.
Auto focus with the kit zoom (the only E lens I have) is quick and efficient,...in two months I have shot several hundred shots and am yet to have a focussing failure. However, I keep to just one centre focus spot and use it wisely, also I only use the kit zoom at f8. I doubt the focus speed would beat your Nikon but for 99.999% of subjects it does not matter. The only really noticeable lack with the kit zoom is that there is no depth of field scale/guide which I often find very hard.
The image quality is excellent and similar to that from a DSLR since both have same size/type sensor.
I would hestitate to describe the NEX as being suitable for the 'camera enthusiasts' that I know, but HAVE suggested/recommended it to two other photographers.
As regards iso and how much pushing one can do,...I have not really "pushed" it enough to find out yet but I can say that the performance at iso 1600 in prints at A3 size is staggering!
THIS is 1600iso;
NEX-3
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18mm
f/8.0
1/500s
ISO 1600