There isn't a better monitor that size for anything close to that price - it's better than a # of monitors (AOC, Amazon branded ones, MSI, etc) that are $700. The 4ms refresh is absolutely fine. In fact, I had that monitor and the 34" UW Alienware ($1k) at the same time and kept the Monoprice one. The Alienware was definitely a better monitor, but it was not twice as good and the refresh was only marginally noticeable so I returned it and kept the Monoprice monitor. The difference I think was the IPS panel and the build quality on the dell was definitely better.
In fact, for $350 there has never been a better priced monitor available that I've ever seen.
It was a screaming deal at $500, for $150 off it's the best monitor deal I've maybe ever seen and looks absolutely awesome. The only negative for me is the HDR implementation is awful, but it's awful on every monitor I've ever used/owned.
The only reason I'd ever get another one is a slightly larger screen with 4k but keeping all the other features. UWD is also fantastic, and this is a great cheap way to experience it on one of Samsungs best made panels.