I really like it because it's fully and truly integrated with KDE, not needed a whole sort of patchs to integrate to it like Firefox needs to.
It's actually great. Not sure about the "Qt doesn't upgrade WebEngine often enough" in other comment (what is "ofteh enough"? I got several updates for it over the year) and of course it can be lagging from stuff from mainstream, but I think for 99% of users it's just fine.
Granted, you can't use Chrome/Firefox plugins, which may it seem not worthy to some people, but there's a basic adblock and greasemonkey extensions shipped with it with default which blocks most of stuff and even you can install a script to speed up youtube ads so that annoying ad will run out in a sec or less. Apparently you can write your own plugins for it but last time I wrote to one of its devs the api wasn't even documented.
There are some quirks on it, though, like the user agent thing - I set up as the latest chrome user agent for every website except accounts.google.com where I left it as the one shipped with Falkon so it lets me sign in, and yet it shows a warning about "upgrading" to another browser.
Ironically, such "warning" also shows up when browsing discuss.kde.org. Yes, the very KDE discussion board warns you against using KDE's own web browser.
Since some years ago I have a silly idea about a plugin that transforms tabs into some sort of Vim buffer list thing that can be filtered by the url bar, but am too incompetent about C++.