Tried the 4k mode today. It works OK. Scaling artifacts are noticeable close up but it definitely is a your mileage may vary. I asked a non-technical person what they thought (after seeing the 'real' 5k) and they said it looks fine. Then they looked closer and they said they could see some not perfectly crisp text. But when just doing your work like coding, writing, surfing, it's not really noticeable if you aren't looking for it. But if you do design work, etc, it's not ideal. A true 4k signal on a 4k monitor would look better. If you're picky in any sense of the word, I don't think 4k mode is going to make you happy long-term. I also don't think that most people spend $1000 on a monitor and settle for a picture that they can "just live with". They want a near perfect retina picture I would think.
If I had no laptops that were capable of outputting 5k, I'd definitely buy the 21.5" 4k ultra fine.
I am thinking of taking the thunderbolt 3 to 2 adapter back because I don't think I'll end up connecting my older laptops to the 5k and accepting the quality of the 4k signal.