Picked up a TCL 43S515 to replace the Seiki SE42UM I've been using as a monitor...
I've been using the Seiki SE42UM for about 2 years as a monitor, it was dirt cheap when I grabbed it, and with the firmware update Seiki put out that more or less fixed the latency issues with it, I've been okay with it. It's not great, I used it as a monitor + a display for both a PS3 and PS4 Pro, and it freezes up occasionally switching inputs and loses all it's settings if there's a power outage, which isn't ideal. But I've been soldiering through because 95% of the time, it's fine, if somewhat a bare bones experience (still not convinced it's full 4:4:4, I'm not sure if anyone ever got a definitive answer on that).
A few weeks ago I saw Best Buy put the new TCL 43S515 up on their site for pre-order, it's the follow up to the 43S405, which for the price, people loved. The S515 offers a decent HDR10 signal for gaming, 4:4:4 chroma, super low latency input (8-10ms), and it's an extra inch bigger on the diagonal with a much thinner bezel. It's $350, or you can get the s405, which is extremely similar, for $250-$300 if you shop around.
My thoughts: It's pretty nice. I mean, it's a $350 4K HDR set, so I knew to temper my expectations out of the gate, but the image quality and response time as a PC monitor is outstanding, and the HDR output for games like God of War on the PS4 look really good, with texture work looking more natural and realistic. There is some light banding in the sky that I noticed in Horizon Zero Dawn with the HDR on, I don't see it in God of War, so I'm not sure if it's the TV or just the implementation in-game. The TV is never going to get called out for being too bright, but I use it in my office and it's generally darker, so it's plenty bright for me. I can't speak to performance in a brighter room or outdoors through.
The built-in Roku OS is kind of the real star of the show for this TV when it's all said and done. It's fast, easy to set up, and each input selection on the screen has a real time preview of what's on the screen of the selected input without changing over. This was convenient when I was installing Demons Souls on my PS3, since I was able to look at the input screen and see how far the install was without completely switching over, a small thing, but pretty cool regardless. It also has all of the Roku app support a dedicated Roku box has, so that's pretty cool as well.
Overall, really happy with the TV, both as a monitor and as a console game display. Not sure it's worth the premium over the s405, but I figured since I was buying anyway, I might as well get the latest iteration.