I just bought a LG 34WK95U 34" 5120x2160px, coming from a 40" 4K Philips display.
I bought it because of the beautiful panel with great colours, the wide aspect ratio as well as the clean bezels.
I got it mainly for photo editing, otherwise I probably would've gone with one of the cheaper curved displays. It's my first time with an ultrawide after having used both large 40" displays and multiple <27"-setups and a day in I'm really starting to like it.
The big downside I've found with the monitor is the stand which is absolutely terrible. It's so weak and wobbly I'd advice against this monitor unless you're planning to use another monitor arm. Absolute joke considering the price.
I was a bit worried about the performance after reading this forum and a few others but everything looks smooth to me so far. The GPU is at 14-18% usage no matter which scaling I use, occasionally spiking higher when there's a lot of stuff going on. Not seeing much of a difference between native res, 2x and scaled, maybe a few % higher average GPU useage at most.
I'm currently using 3008x1269 but I've been bouncing back and forth between 2560x1080 and 3360x1417 as I haven't made up my mind on which resolution to keep yet. 5120x2160 is too small and 2560x1080 lack screen real estate.
Now I'm not coming from a 5K iMac so I can't compare performance and font sharpness to that display.
I do get lag in some applications however. Photoshop is real sluggish for example. Even iTunes, when scrolling quickly and displaying large amount of album art. I think it's because of the 8 GB ram which is constantly at 70 - 85% use. I'll upgrade to 32GB today (and I hope) and suspect it'll make a big difference. I have the i5 and the CPU and GPU almost never went above 30% usage from my workload yesterday. I've yet to do heavy Lightroom work though.
UPDATE:
Ok, I upgraded to 32 GB RAM and it's a massive improvement. Memory usage is now at 25% (down from approx 75%) and GPU usage is down to about 8% (down from approx 18%). All the lag I previously experienced seems to be gone! I used this ram:
http://eu.crucial.com/eur/en/mac-mini-(2018)/CT13492047