I'm going from owning Plus models to the X. I've done my homework and know that the display (excluding bezels) is about 1/4" narrower than the Plus and only a hair wider than the non-Plus models. Useable display area is definitely much closer to non-Plus than Plus models. There have been some detailed analyses posted online since the keynote that clearly demonstrate this.
However, I still feel most people (even those coming from Plus phones) will end up with the overall impression that they have an improved display in front of them. The narrow bezels, OLED, Tru-Tone, and much higher DPI than the non-Plus phones will make the display feel very premium. It will be terrific to look at.
The reduced width will take some getting used to for web pages, viewing or editing documents, etc. I don't see it as a huge change, but for those people highly focused on maximizing the display size, the X may not be a good choice. Personally, I like a roomy screen but it's not the top priority and I'll get over it. I still want the X for many other reasons. I have an iPad Pro and a Mac when I really need more display.