Here's my experience with 27" 4K monitors.

I originally planned on getting a single 32" 4K IPS monitor, but got tired of waiting for it to come out in Canada.

I test drove a few smaller 28 4K screens in the local computer store and realized that I could easily live with 1:1 on a 28". I ended up getting a Dell 27" 4K monitor because I wanted an IPS panel.

I realize I'm in a very small minority of users who can use 4K with no scaling at 27" -- I use a standing desk and I have single vision computer glasses for the distance my monitors are away from my face.

When I first got the 27" 4K monitor, I was in awe of the glorious real estate it afforded me. That awe didn't last long, however, because I ended up wanting even more real estate. So I bought a second, matching 4K monitor, and it's great. It's like having 8x1080p monitors.

I would love the additional real estate from a third and fourth monitor (i.e., 8K equivalent) but things would get harder at that point because of neck movements, stand availability and the fact that I have a small form factor PC with no room for additional video cards.

I tried a 28" (TN) 4K, an Acer B286HK. Horrible ghosting and capacitor whine - if I had Wikipedia open you could hear the capacitors scream from across the room, and lines would blur into each other while scrolling. I eventually replaced it with a Dell P2715Q (IPS) too, and that's a really great monitor.

All in all though I would have been better off going 1440p at 27". I need some DPI scaling that probably gets me back to 1440p anyway, and it would be easier to drive in video games.

So that's what I recommend now based on personal experience. YMMV, but the Crossover 2795QHD seems ideal for productivity work. The Acer XB270HU or XF270HU are ideal for gaming, but adaptive sync is unnecessary for productivity so you might as well get a Crossover instead.

I do agree that 1440p is probably best for most people. Most of the people who see my setup just ask "How the hell do you do that?"

I'm one of those people who has a bad habit of having 50 windows open at any given time. Sure I could change my habit, as I actually "only" need tiles of about 10 screens at any given time, but so far, having two 27" 4K monitors side by side has improved my productivity significantly.

I originally had them stacked vertically, but I changed the orientation to side by side landscape when I changed desks, and I found the new wide orientation works even better for me. Having needed reading glasses for the past few years also helps, because the glasses I have magnify the screens a little bit too, so it helps.

28" 4k panels are around 150 DPI, which is only 50% more than the "standard" 22" 1080p panel. 32" 8k panels, though, are going to be 275 DPI.

It is all personal experience, but my 21" 1080p monitors have awful pixelage that is very noticeable at standard view distances. Obviously, it has pitifully low DPI. Reasonable desktops (modern Gnome, latest KDE / Qt) now fully support DPI scaling, so there will be no problem going forward for people to use much more reasonable fidelity displays going forward because its not 2005 anymore, and we don't have to be bound by some awful fixed DPI in Windows that is incredibly low.

I bought a 4K monitor last year, and since it was about as expensive as all the good 24"-28" models, I went with a 40" model (the Philips BDM4065UC, to be exact). And it's amazing. 4K is awesome when you have the screen area to actually fit more content on the screen without having to Alt-Tab all the time (e.g. code + API documentation + test results). It's basically four 1080p monitors in one.

I never used something above 21/22 IIRC, but as a secondary monitor. I'll give it a shot, although nowadays I'm having a retro fetish, give me a 10" orange / black CRT and I'll be happy.