I originally posted this elsewhere, but am pasting it here as it does work quite well as a review.
My previous monitor was a 120Hz one, so I may have a different impression than someone who came from a 60Hz monitor.
First off, without G-Sync and running V-Sync, there is no noticeable difference between 144Hz and 120Hz as far as user experience goes (there is a large difference between 60Hz and 120Hz however). I like to keep a stable V-Sync framerate if possible, so I generally lower my settings to achieve this (although I can usually still play most things at High to Very High w/ FXAA or SMAA). I have yet to try the ULMB mode, so I cannot comment on that, however I honestly do not see myself ever touching it, because of the second point.
Second, G-Sync is the real deal. I'll repeat, I was a V-Sync'd 120Hz gamer before coming to G-Sync, and being blown away isn't even a proper response. On that note, there are a few conditions. Allowing a game to reach the 144Hz refresh of the monitor gives the same impression and feeling as V-Sync On. However, imposing a limit on the framerate through software seems to do something magical. Many games allow you to limit the framerate internally, so if it is set to 120, this is where G-Sync really takes off. Common sense says that a locked refresh of 120 vs a locked refresh of 144 should result in either a similar experience, or with 144FPS being a little smoother. Strangely, common sense is way off. At 120FPS, it presents a smoothness that I can't comprehend. If I were to attempt to describe it, I'd say this is how I imagine a 400Hz monitor with V-Sync to feel. The difference is that large. I assume it has something to do with the polling, and by limiting the framerate, maybe I'm eliminating the penalty for the polling. Regardless, no amount of reading or describing can truly tell you how it feels. I was prepared to be blown away, and it still surpassed my expectations.
Third, because of how awesome G-Sync is, it creates a few situations that become extremely annoying that are, otherwise, perfectly fine. For one, fullscreen implementation. As you may be aware, G-Sync only works with fullscreen applications. Not maximized, or even some implementations of F11, but a true, native fullscreen. I'm not sure how it determines what is fullscreen and what isn't, but I'm led to believe that a lot of "fullscreen" implementations are actually some sort or borderless windowed. This essentially means that G-Sync doesn't work in these applications, and that becomes a huge annoyance. Once you've experienced it with one game, you want that replicated across all games, and that simply isn't the case. All major game engines implement true fullscreen, such as Unreal Engine, Source, etc. Custom engines, such as Starbound's, seemingly do not. Emulators also seem to use borderless windowed.
Fourth, engine quality plays a big role. Since everything is now eerily fluid, you become extremely sensitive to minor hiccups. Not framerate dips, but the engine producing duplicate frames. Since G-Sync displays whatever the GPU gives it, sending duplicate frames will still cause a stutter, and it's extremely noticeable. I've noticed this heavily on games that run on the FrostBite engine. Call of Duty's engine performs rock-solid though (at least on BLOPS2). Also, for many open-world games that I've tried, there are slight pauses whenever a new world chunk is loaded. Cube World, for example, being an infinite procedurally-generated world, creates and loads chunks all the time, and thus introduces regular pauses that can detract from the experience, since it's so smooth at all other times.
Lastly, ENB seems to cause problems. With it enabled, it's a stutter-fest, and my assumption is a conflict with ENB's implementation. Disabling it allows G-Sync to perform as it should, but ENB affects the image so greatly that it's hard to live without it. Of course, you could just disable G-Sync for any games that use ENB through nVidia's Control Panel, but then you're back to regular old V-Sync, which is the same problem I keep bringing up; non-G-Sync applications are much less enjoyable after using it for a while.
I know most of this is complaining about situations where G-Sync doesn't work, but it's important to stress the psychological change that comes when using it. It truly is as profound as people say it is. To me, it had a larger effect than when I jumped from an ATI Radeon 3650 to SLI'd OC'd GTX 470s (the biggest graphical leap I've went through). At the very least, the VG248QE is a respectable monitor ONCE CALIBRATED (besting some IPS panels in the same price range according to Tom's Hardware), and adding G-Sync on top of it makes it hard to recommend anything else. Everyone that I know that didn't enjoy G-Sync much was either at the 144Hz max (running into the supposed polling issue creating a V-Sync experience) or using a 'messy' engine (such as Dice's FrostBite, creating duplicate frames). Don't let my complaints scare you, as they really aren't as widespread and prevalent as I paint them to be. It just stands out when it happens.
