So I got the AW3420DW a week ago. It is without a doubt the best monitor I have owned. Though I will say going from 27" 2560x1440 144hz to 34" 3440x1440 120hz isn't a massive upgrade. My old monitor was the ROG Swift PG278Q. It was an excellent TN panel that served me well. I wanted though to try ultra wide so here I am.
So to my surprise I have zero flaws with this monitor's build quality. No dead/stuck/bright pixels even though it is an IPS panel I have very low ips glow that isn't noticeable. No ghosting to my eyes. One complaint is it might be a bit over saturated at stock though that just might be me going from TN for many years to IPS.
Gsync worked right off the bat. I plugged it in and after a few seconds Nvidia detected it as a gsync monitor. I have't notice any difference between 144hz of my old monitor to 120hz on this monitor. Performance impact of going to 3440x1440 over 2650x1440 isn't that much. Some games I had to lower some settings most most games run just fine maxed out on my 1080 Ti. Sure I am not getting 120 FPS in every game but I don't care and FPS games where FPS matter like CSGO run at like 240 FPS anyway.
The monitor is as bright as my old monitor and I keep it at max brightness. If I am viewing a mostly white page it is a bit bright but I prefer the better color quality at max brightness. Going from my old monitor to this nano IPS the colors are indeed a huge upgrade. My friend I had over also was wowed by the colors and enjoyed playing games or even watching videos. Once you get a 21:9 movie or show it looks great.
Speaking of 21:9 I found it hasn't given me that much of an advantage over 16:9 in competitive FPS games. I do notice that if I am camping a spot I do get a much wider view so I can watch for movement and cover more so there is some advantage. So I can be guarding one hallway but still be able to see other hallways or doors. It doesn't help much though when you are moving around a lot and pushing an objective. Don't think you will get a massive advantage. For strategy games and third person games are so freaking good on this monitor. I can get to see so much more info and less clutter for strategy games like HOI4. It is truly impressive.
Though there are some negatives with 21:9. Some games don't support it that well. Like the Witcher 3 where cutscenes and inventory/merchant screens will have black bars on both sides. There is fixes but it takes editing some files. Watching media there is a lot that are not in ultrawide. Most of youtube for example is not and often Netflix, Prime Video, or Google Play are not in 21:9 so you will see black bars on both sides. There is an extension in Chrome to force a video to ultrawide but I don't like using it as it makes everyone look fat.
It is also curved which took a couple of hours to get used to. At first I didn't like it but it helps with immersion.
Overall I would say this is one heck of a monitor. I would have liked it if it had HDR but that is far too expensive at the moment.