Just received my U3223QE yesterday. So far so good, I'm coming from late 2014 27" iMac, so I was very worried going to a non-retina screen. So far I'm quite happy, I'm running at "looks like 3360 X 1890 in scaled". I was really worried the txt would be not sharp and or too small. But its really good. I'm using it with a 2018 13" MacBook Pro, (waiting for the MBA refresh hopefully soon). I am a Tech investor so I don't do any video editing or graphics work. For me, the monitor is really good. The only hiccup so far is that I cannot get power from the USB-A plugs in the back of the monitor. I'm currently using the single USB-C connection so able to get power from that, but I'm unable to get power on any of the other connections. In the Manual it seems to say that I can get power from these inputs.
You cannot imagine how relieved I'm that the monitor looks good, I have spent hours on this forum and other reviews looking at the 5K2K LG, the 5K LG's, the Apple Monitor and other monitors. I was all set to get the Apple Pro Display, but every time I felt that it was really expensive, which it is and it's really not necessary for what I do. I do spend a lot of time in front of the monitor so I was willing to pay for something that was good. All of my other device are retina (5K iMac, MBP, 12.9" iPad Pro, iPhone). If you've been waiting to get a monitor I would say these Dell UltraSharp 4K's are very good.