I'm spending a fair amount of time remixing to ATMOS these days. While mixing, I'm able to toggle between the original stereo mix, 5.1, 5.1.2, 7.1, and 7.1.2, and 7.1.4. In my view, the experience gets incrementally better as I move through the formats, finally ending at 7.1.4. That being said, if there's no program material up high in the space, there's no difference between 7.1 and 7.1.4, for instance. And, there is an argument (which I sometimes ignore) for not putting any material in the overheads, as if the music is played in final form on a non-ATMOS surround system, anything placed in the overheads is lost....
As for 'what's the big deal with ATMOS', it's clearly that the program material plays coherently (in theory) across all the ATMOS- supported surround formats... that is a big deal.