With their high pixel count and sharp, crisp pixel densities, 4K monitors can be brilliant tools for everyday desktop work. They provide plenty of space for looking at several desktop applications at once, and they also make it much easier to see and juggle multiple windows in creative applications such as Adobe Photoshop and Premiere Pro. If you regularly use these kinds of apps in your everyday work life, 4K monitors are almost definitely going to be worth the extra investment over lower resolution displays.
That said, if your main reason for buying a 4K monitor is to simply play games at a 4K resolution, then you'll need to make sure you've got a powerful enough PC to really make the most of it. This can end up being quite the costly endeavour, as you'll need one of today's best graphics cards inside your PC in order to play them at a decent frame rate. We recommend at least an Nvidia GeForce RTX 3070 or AMD Radeon RX 6800, although if you want to play at higher quality settings, then you really want either an RTX 3080 or RX 6800 XT. Under normal circumstances, these cards will set you back at least £500-700 / $600-800 - as much as your monitor in some cases - although given the current hardware shortages taking place at the moment, you're more likely looking at £1000 / $1000+.
Ironically, the cheapest way of getting a full-blown 4K gaming system in your house is probably buying an Xbox Series X or PlayStation 5. As we found out when we tried to build an equivalent Xbox Series X PC, we would have had to spend almost £1500 to get the same kind of 4K gaming chops as the Series X, which is over a grand more than what the console costs. In this sense, the question of whether 4K gaming monitors are 'worth it' depends very much on how badly you want to play games at this resolution, and how much you're willing to spend in order to get there. As someone who regularly plays games in 4K, they do look absolutely lovely and sharp at this resolution, but I wouldn't say they're noticeably different from playing them at 2560x1440, for example.
Of course, there's nothing wrong with buying a 4K monitor and then using it to play games at 1440p. If anything, it gives you even more flexibility than lower resolution screens, especially if you're in the process of trying to upgrade your PC at the moment, or you're waiting for graphics card prices to come back down to normal again. Personally, I think 1440p is still the ideal sweetspot for high-res gaming in terms of cost and value for money, but as I mentioned above, 4K monitors are much better for everyday desktop tasks, and if you've been working at home for much of the last year and are likely to continue doing so, then you'll thank yourself later for getting a screen with more pixel real estate.