Charlie Osbourne: I have a 55" Samsung 4K Smart TV in my lounge, purchased around 4/5 years back. It's been reliable and with decent image quality -- although, sometimes, it does have a tantrum and requires a hard reset to reconnect to HDMI-based services. I also wish it had more HDMI ports, but buying a dongle for the back helped. The sound quality was terrible and very flat so I ended up buying a Bose soundbar to go with it.
Next TV? I'd like to upgrade next year. If money wasn't a factor, I love the look of Samsung's Infinity screen models. However, more realistically, I'd stick to a TV by Samsung and go up to a 65".
Marc Wojno: I have the 65" Samsung QLED 4K. The quality is excellent for programs produced in 4K, such as streaming services, with gorgeous color and sharp detail. As you would expect, it does accentuate the limitations of programming not in 4K or HD. I'm impressed with the TV's upscaling capabilities. My litmus test is its ability to play composite video. I'm happy to say it does this better than I expected (I can actually watch my laserdiscs without being distracted by the limitations of the analog image).
One big issue I have is the slight delay of the dimming zones, especially with white titles on dark/black background. Such titles tend to fade in fast when they should be snapping up (think of the opening and closing titles in 2001: A Space Odyssey)...can't stand that. Overall, I'm pleased with the TV (although I sometimes catch myself missing my Panasonic 1080 plasma).
Next TV? As you would expect, I'm waiting for Samsung to improve its local dimming zone tech so that cuts are cuts, not fast fade-ins. Put that ahead of expanding to 8K. Also, upgrade the speakers and audio features on the TV so I don't need to spend extra on a soundbar.