I bought one last week on sale for $400, on the heads-up on the front page. tl;dr: It isn't a hardcore gamer's monitor, and it isn't perfectly accurate (colors/gamma), but if you're trying for something pretty good at both on a budget it's awesome.
27", 2560x1440, 144Hz, and decent IPS or VA is roughly the featureset I've been looking for, and I've mostly just been waiting for it to drop to a reasonable price. The trouble is that the usual displays fitting that description all use the same AUO panel and have terrible quality control problems (especially the Acer). QC in this case mostly means backlight bleed, IPS glow, and white uniformity. This Nixeus uses the same panel, but is both cheap and by all reports much better at quality control than its competition at that price.
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144 Hz is the best thing since the return of mechanical keyboards. If I had to go back to either 60 Hz or a mechanical OS drive, I'd have to think very hard about which one. Aside from more fluid motion, the latency reduction is real, and it doesn't even take gaming for the benefits to clearly show up. Computer use is just that much more comfortable and responsive now.
This monitor does not have particularly quick response times, possibly due to overdrive issues. Others report that only overdrive=low works but it doesn't matter because low is the ideal setting. On this one, changing overdrive settings doesn't seem to do anything at all, and all settings allow a clear bit of blur that isn't there on an MG278Q (which is very similar but TN). If I liked my gaming twitchy and highly competitive, this would be a problem, but I mostly don't and it isn't. I also find it hard to believe that going from even this possibly subpar 144 Hz experience to TN + ULMB at 144 Hz could be half as big a difference as 60 -> 144 Hz itself.
I can't test Freesync due to having a GTX 960, but tearing never bothered me anyway. It does have a full 30-144 Hz Freesync range, which is a nice positive.
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I don't have anything resembling proper calibration hardware, so the following is all by eye using appropriate test patterns, such as these
Gamma is just a touch bright across most of the range, meaning 2.1 to 2.15. Both the dark and bright ends may be slightly compressed, but not enough that I could reliably tell. There is a gamma adjustment in the OSD, but using it turns greys blatantly orange, so something's clearly broken. It only operates in increments of 0.2 anyway, so it wouldn't be terribly useful. Dark end compression may be fixable by setting color effect to photo instead of standard, but that looks like too aggressive a fix and causes an obvious artifact on gradients.
At one point I made my own full-screen gamma test pattern, which turned out to be very interesting for reasons other than just gamma. First, the nominal results: gamma is close to correct on the right half of the screen, but less so further left. The left side of the screen is very slightly cooler in color temperature than the right, independent of gamma. The gamma makes greys oh-so-slightly warm, possibly to compensate for gamma making greys cooler as the viewing angle increases.
Now, the first weird thing about this test pattern is that the portion with a white horizontal line every 8 pixels on a black background hurt my eyes, and the portion similar but with 4 pixel spacing somewhat less so. Pixel walk? Something like that. Lagom's pixel walk testing shows it does exist, but it's very subtle. Test patterns 6a and 6b over there both hurt my eyes in the same way, though, and pixel walk itself might be tougher to see just because this is 144 Hz. Something on those must have been a fairly impressive strobe. The second piece of weirdness is that I had the test pattern open for a while, and when I closed it, the fully white parts left fairly strong persistence on my dark grey desktop background (but not on black terminal windows). It went away in a couple of hours.
The first problem seems to be well fixed and the second seems at least substantially helped by turning contrast down to 40, at the expense of a touch more IPS glow relative to the image's brightness. This seems to have no other ill effects on image quality, and doesn't affect gamma etc.
The default color temperature (sRGB mode, RGB 128/128/128) seems a touch on the cool side. 6500K mode is pink. In user-defined mode, RGB 128/127/126 seems about right to me, though the margin is slim enough that I'm not sure why I would have a valid opinion. Custom PC Review's calibrated settings of 128/127/124 were clearly not right for my panel. A separate option called low blue ray is clearly just reducing blue pixel values as if adjusting color temperature, and it looks like garbage.
The leftmost ~1 cm of the screen and corners appear to get a touch less backlight than the rest. It's a fairly tiny effect, both as a percentage of screen area and as an amplitude, and it follows a smooth enough gradient. I can't distinguish any other white amplitude nonuniformities.
Only on grey tones, a very faint ~5 mm by ~10 cm horizontal line is visible out in the middle of the screen. I probably would never have noticed it if my desktop background's shade of grey weren't the perfect one to make it show up. The tones that make it show up are the same ones that make persistence show up, but it doesn't match up spatially with anything I do and doesn't go away.
It has no dead pixels.
Backlight bleed and IPS glow are both evenly-distributed and not excessively heavy. The net black level / contrast isn't going to win any competitions, but I've seen much worse, and this one is perfectly acceptable to me. The following picture was captured at ISO 1600, F2.8, and a quarter-second shutter - enough to make it look much worse than it does in person with any kind of reasonable room lighting. The white in the corner is one pixel of the cursor that's still on screen.
Now, since I just spent a wall of text describing a lot of flaws in this monitor, I've got to reiterate that it's pretty good overall and I'm very satisfied with my purchase. Most of those flaws are somewhere between tiny and completely irrelevant. No TN panel can touch color accuracy like this. 144 Hz is the best. IMHO, both at once at 1440p for $400 could really only be damped by bad quality control, and here's one more datapoint saying it's at a non-problematic level.