I saw an advert for this device, and it intrigued me. And, since it wasn't too expensive, I thought I'd indulge my curiosity.
It's a simple interface device to connect your Samsung phone (S8, S9, perhaps others, too), to a HDMI monitor, and a keyboard, and mouse. While you can already use the USB port to connect a keyboard and mouse, directly, this gadget lets you connect an ordinary monitor, too, and not have to mess around with screencasting.
So why would you want to do this?
Perhaps you're a travelling worker who uses their phone for all sorts of work. Then comes back to the office and either has to carry on using it, with a tiny screen, to continue on their tasks, or has to sync their phone with a computer to transfer the information. With this gadget, you can carry on using the data on the phone, without the inconvience of a small screen, tiny keyboard, and no mouse. The phone has the computational grunt of some laptops, so it's a reasonable thing to keep on using it.
Or you'd like to watch videos from your phone, on a normal sized monitor. You can plug it straight in, no need to try screencasting (which can only be done using WiFi—so requires a smart TV or a dongle, not to mention all the fun and games of configuration, or failures due to bad WiFi).
So those kinds of things (working on the phone, viewing videos on a big screen) were foremost in my mind. Particularly as I'd discovered the pain of trying to get files off the phone to work on them with another computer. The USB connection liked to fail after transferring a few files, or even trying to list a directory of photos. Sending files via Bluetooth was similarly unreliable. Trying to transfer files to a Mac was a nightmare, it's not supported, at all. There were apps to do an entire phone backup to the computer, but that's a ridiculous time and space wasting exercise when I just wanted to copy a couple of files. And it didn't want to do hacks to the phone to get filetransfers working.
On that note, it seems to be that you're expected to shovel all your files onto a cloud service, and work that way. Well data isn't free, having to barrel files through an external service when my devices are right next to each other is not efficient, nor cost effective, nor a particularly secure thing to do.
Samsung has two Dex products:
The older one looks like a round ashtray, that you sit the phone into, completely blocking off the headphone port, and the mike and speaker at the bottom end of the phone. Though still leaving the fingerprint scanner accessible on the rear of the phone, and the camera can be aimed in your direction for face identification purposes, but probably mis-aimed for teleconferencing.
The newer one is a flat bed you put the phone on top of. And the phone can even stay inside some of the Samsung covers while connected. This leaves the headphone port accessible, as well as the mikes and speakers. But the fingerprint scanner is completely covered up, though there's enough slack that you can get your finger between the camera and Dex pad (the USB connection is on a flexible rubber mount, so there's some play without forcing things). But if you want to use the camera, you're going to have to get creative with something else to prop the gadget up into position. The Dex pad gets in the way of the rear camera, so it would have been a good idea if the front camera was automatically selected when docked.
I plugged things in, and it just worked. I could watch and listen to files on the television set. I could reply to some emails a lot easier, though the two supplied email programs (from Samsung and Gmail) are an absolute disaster for replying to messages with selective quoting of the prior message. I participate in things where quoting the entirety of a prior message is completely unacceptable, and not quoting anything is not really appropriate, either, so it becomes a copy and paste headache, or a retyping headache. Which is not helped by the rather appalling cut and paste interface on Android. It's difficult to even get it to highlight some content on the page, to begin the attempt.
While 1920 by 1080 is a reasonable screen definition for viewing videos and pictures, the phone doesn't do a good job of rendering the user interface at that resolution (the icons and text). They look decidely low resolution and clunky.
There's going to be a bunch of apps that are not going to take advantage of a bigger screen, they'll just appear in a fixed size, portrait style, window. That didn't turn out to be too much of a problem, for the few things I tried that behaved that way. It did mean that you could open two windows side by side, and do one thing while reading another.
There'll be some apps that'll just refuse to work. Firefox, for instance, refuses to run, giving a brief error message about it only being designed for touchscreens.
Things that use a swipe action are awkward to do using a mouse, though you can keep the touchscreen active and still swipe on the phone. But it's not that intuitive to do, anymore, when you're looking at another screen while swiping. And I had some shenanigans with the phone being unexpectedly in landscape or portrait mode while sitting flat on the desk.
The phone only connects to the pad via the USB connection, there's nothing to hold it firmly in place. It readily skids about, which could be a point of mechanical failure. You'd want a cover with some grip to it, or to slip a non-slip pad between the phone and Dex pad. Particuarly if you're not going to leave the phone sitting untouched on a level surface.