First impression: Looks really different than the Nano 9 but feels eerily similar.
Although it looks like a completely different shoe than the Nano 9 but don't let looks fool you, the Nano X doesn't diverge too far from the 9 feel-wise. Using a similar midsole/sole to the 9, the Nano X differs mostly by the upper.
The height of the shoe definitely stands out - just looks taller than the 9, especially the heel collar. But it's not uncomfortable or unwelcome. In fact, I've grown to really like the added heel collar height.
The changes to the upper also make your foot feel like it's more locked in. The extra padding in the heel and over the midfoot give this shoe a more snug feel - but not too snug where I'd characterize it as narrow. The shoe also feels more stable with all that extra padding/material in upper. I also found it does well running.
Overall this is a solid shoe and a great update from Reebok. Reebok stuck with what works with the 9 and made it better.