I also have one, and it's holding up pretty well. A month or so ago I broke out my colorimeter and it had almost 100% sRGB at around 120 cd/m2. I don't recall the delta E, but it was very low.
While I didn't measure the backlight, it does seem to not go as bright as before, judging by the levels I set in the OSD. I never went above 70% or so when the sun was shining in the room (not directly on the screen, though), so it didn't have any effect on me.
I understand there are two version, I have the second one. But I don't think there's a difference in the panel itself, I think the change was related to HDMI support.
I can't comment on the latency, the only games I played on it were Civilization and Anno Something. Never had a problem for this.
I have been using P2415Qs for over 10 years by now. Replaced some, bought second hand, had to ship to Dell at one point because of the wake issues (pointless: they never really fixed them), so I know the drill. There are actually 4 versions.
The last new one I bought in 2018 I actually paid the same price for it than when I bought my first one in 2015, so it is one of those few computer accessories that significantly increased in price over its course rather than decrease.
If you cannot see the P2415Q degrading and/or being generally crap in any metric (EXCEPT DPI) when compared to even the non-IPS black Dell monitors from this decade, you are simply blind. They are early HiDPI-revival-era panels, and it shows.
Some of the newer IPS black panels are so good that it is tempting to just take the DPI hit and go 27''... albeit with care as it seems Dell has decided this last year to put some filter that further increases blurriness.
I don't pretend that I have the best vision out there, though I don't think I'm completely blind since I don't run into things. But I actually measured this display, and it's within specs. So maybe both I and my colorimeter are blind? Sure, it's not absolutely impossible, but how likely is it?
I actually have a newer "ips black" dell, an ultrasharp 3223qe and yes, it's much better.
But what I'm saying is that the old one is still good. However, I never pretended it was as good as current models. That's moving goalposts. The initial comment was about the display degrading, so comparing it to itself when new (not even other similar models from that era!). Mine only seems to have become somewhat dimmer, but not enough to matter in my day-to-day use since it's still brighter than I need.
I also have one, and it's holding up pretty well. A month or so ago I broke out my colorimeter and it had almost 100% sRGB at around 120 cd/m2. I don't recall the delta E, but it was very low.
While I didn't measure the backlight, it does seem to not go as bright as before, judging by the levels I set in the OSD. I never went above 70% or so when the sun was shining in the room (not directly on the screen, though), so it didn't have any effect on me.
I understand there are two version, I have the second one. But I don't think there's a difference in the panel itself, I think the change was related to HDMI support.
I can't comment on the latency, the only games I played on it were Civilization and Anno Something. Never had a problem for this.