So, I've made the move to a 4K display for my Mac Pro. That's a lotta dots. Well, a 4K TV, really, but that's only because there aren't any reasonably priced monitors.
There are definitely both pros and cons to this.
There's really only one, but it is a big one: A huge amount of contiguous desktop space becomes available without being broken across multiple monitors. This is a huge benefit for me, as I tend to have many applications and shells open at once, and I also regularly create and edit high-resolution data like DSLR images and printed circuit boards.
It is worth noting, however, that this is really only useful because I chose a 55" display; a "normal" size monitor would present such small text and features that it would be useless in terms of giving you access to the available resolution, so the only way to usefully use 4K with a (relatively) small display is to make everything really large — text, images — at which point you might as well have just used a lower resolution display in the first place.
The worst thing is OSX's "one menu bar on top." That menu bar is very far away from most everything on the desktop now, and mousing to it is really annoying, particularly as OSX 10.12.6 doesn't offer proper mouse acceleration. I find myself longing for the Windows/Linux style of "app menu lives on app windows" menu interface. Unfortunately, no such capability exists within OSX. So the workaround I've found is to depend on keyboard shortcuts; move the focus to the menu bar (on my machine, SHIFT+⌘+F2 does that) and then use the ←, →, ↑, ↓, and Return keys to navigate and select the various menu commands and options. This is quite fast, and I don't exactly mind it, but as yet it's not a habit and so I'm not yet as happy with it as I probably will be after enough time to get used to the idea. I also use ⌘+tab and ⌘+` to switch applications and application windows quickly.
The next issue is speed. This is likely a consequence of the graphics card I was forced to use (a Radeon RX460) because OSX 10.12.6 doesn't support a lot of 4K-capable cards, and Apple, in their (cough) wisdom, no longer supports my 12/24-core, 64 GB, 2.9 GHZ Mac Pro with new OS upgrades. So I'm stuck. Likely others aren't and so a newer graphics card might make this point irrelevant for them. It's not as bad as it might be for me, either, because I don't game on my Mac; I'm pretty much a 2D graphics user. Just something to be aware of.
How good your vision is will definitely impact how well you like this kind of thing, or not; the difference in distance from the center of the display to the edge is considerable, and if you rely on glasses to correct near field vision, for instance if you've had cataracts corrected with implanted, fixed lenses and so have difficulty naturally changing your vision's focal depth, you may find that it is difficult to see everything at once without actually moving your head around.
Lastly, the TV has a glossy display surface, and so it is prone to reflections. I had to change the physical arrangement of the lighting in my office to get rid of my own reflection in the monitor. The upside to that, however, is that I like the new lighting arrangement better overall.
I was really worried, having chosen a 4K TV (a 55" TCL Roku C110-X) as the display, that viewing angle issues would result in unacceptable brightness, and/or contrast, and/or color variations at the edge of the display. It turns out that the display tech of this relatively inexpensive TV is up to the task; it uses an ips panel, and the display quality everywhere on the panel, even from up close at relatively high angles, is perfectly acceptable to me.
I was also concerned that the "smart TV" stuff, the Roku component and so on, would require me to put another WiFi client on my network. Turns out the TCL allows you to skip all that, so no security issues and no additional WiFi network loading.
I consider this change in my working environment to be a great success. Between the new (well, new old stock) graphics card and the new display, the cost was under $500, and for that I got four smaller HD (2K) monitors off my desk and stopped having to deal with images and user interfaces being cut off by their limited display areas. Overall, this has been a real win for me. If you've been holding off on such an undertaking, I encourage you to go ahead.