Having an Samsung Galaxy S10 and a Galaxy Tab S5e which can both run DEX I have to say that I've only used it once on both. Its definitely cute and I can see the potential in it. But the reason that I'll never use it is that there's no real avenue for it given I have an iMac 5K, a MacBook Pro 2018, tablet and phone where I can always choose the best and/or most convenient device to do something. That choice will never DEX… especially given the display I need for it is already connected to my iMac and I just have to push to on button rather then mess about.
That said, I can definitely see a target market for it. Take my mother for instance, she's got an iPhone X, iPad Air 2 and an iMac 27" (pre-5K). The only reason she's got the iMac is that she wants to see certain things at a certain side that her iPad cannot offer. 99% of that is web browsing. Having DEX (or Apple "equivalent") would enable her to do away with the iMac, and hence having to maintain it and its software, and instead have a display, keyboard and mouse. DEX would be perfect for her.
Apple's Handoff is not really a replacement as Handoff still has the requirement of another device you're handing off to.
Coming back to DEX, the fact of its existence and the form in which is works means that I can simply buy myself a USB-C hub (I already have a Satechi multi-port adapter) and I can connect up not only a display, wired keyboard and mouse but also ethernet, SD cards, external USB storage and all sorts of other stuff really easily. Things that make moving things around quite simple that cannot be so easily done with Apple.
Where DEX really isn't going to do as well as it could is really going to be down to developer support.
PS While many will go on about Samsung "bloatware" there really isn't a lot that comes on top of the OneUI these days. While true of the past its hardly the truth today. On top of that much of the bloat comes from the carrier rather than from Samsung. Hanging on to the past like this serves no purpose just like of an Android person we to go on about iPhones dropping cellular coverage if you "held it wrong".