I just ordered a Nikon D850 body (factory refurb). Moving up from cropped-frame sensor bodies (D7200 etc.).

Yes, the D850 has been around for a long time - but I already have plenty of Nikon DSLR lenses (including full-frame ones) that will work well with that body. Not planning on going mirrorless anytime soon.

I see that the D850 supports an XQD card as well as an SD card. XQD seems to be an expensive niche format which will be superseded by CF cards in the future, if not already - but I'm considering getting a single XQD card (and a PC reader for the card) both to use it for in-camera backup of images that will also be saved to an SD card, as well as for the potential speed boost when offloading images to the PC.

Anyone using XQD cards? What has been your experience with them? What are you using for a reader on a PC or Mac?

Have you set up the card as a mirror for the SD card, or are you perhaps sending RAW files to one and JPEG files to the other card, or some other way of organizing the in-camera storage?

The D850 had a firmware update some years ago that allows you to use a CFexpress instead of a XQD card.
You can find a CFexpress card for less and they are faster. Use that instead.

I like writing Jpg and RAW to both the SD card and the CFexpress card.

I use XQD because that's what was available when the D850 launched.  Zero issues with it.  That said, CFExpress is less expensive and faster (although you won't see much if any of a performance boost on the D850 due to hardware limitations) and are more future proof, so I'd go that way.

No issues with mine and they've been used hard.  I do keep a SD card slotted in, but that's just for overflow at events until I can swap cards.

Thanks - ordered a 128 GB SanDisk CFExpress card and a plug-in reader for the PC…

I have a D500 that I use a 32GB XQD card, I am not switching to CF Express even though it is way better in every respect. My present situation is good enough for me.