Having used Quark from the start, and having tried other page makeup applications, I've always returned to Quark. The typography handling is superb, and the interface easy to get around. I've even seen complete non-technical novices make their first layout without even looking at the manual.
The pricing is extremely generous for the quality. Why would anyone use InDesign apart from "everyone's using it" or "my company only has Adobe licenses" or "I use Photoshop and…" etc. The one-off licence fee for two machines is refreshing in a period where annual subscriptions are widespread. I've published many professional books for clients in Quark, and find the workflow seamless.
I also like Quark's independence, and the fact that it pioneers features that users want. While I wouldn't use every single feature, I always feel there's something new to discover or use and have never felt limited by the software.
The various Quark-related website(s) are confusing and messy, and don't communicate well - please integrate the main site, support and licences so we don't have to sign in separately EVER! Otherwise, support is generally good. There was one bug a few versions ago that caused a crash on moving a text box, but that was fixed. I can't think of a single issue apart from that.
Quark XPress is a powerful, well-priced and independent tool that's worth adopting in your workflow. Make your own decision about the "everyone uses InDesign" hype - "popular" does not always equal "best"!
Professional-level print publishing, and PDF generation for various kinds of output.