We tried reaching you on a private message asking for the Service Tag number to ascertain the warranty but did not receive a response. Please feel free to reply to the private message whenever you are available.

Sleep doesn't work for me -- the system just wakes up immediately. This is true even if I have turned off my bluetooth peripherals (keyboard, mouse) prior to attempting to sleep. 

Hibernate is a little better. Usually it stays hibernated but sometimes it wakes itself up again right away. 

Yes, the inability of a (supposedly) high-end laptop to just go to sleep, when instructed, should be quite an embarrassment for Dell, but apparently they don't care about it. 

OP said s/he is thinking about buying one. Of course s/he doesn't have Service Tag. 

that's sad. I also though of precision 5550 but I guess it has similar hardware and thus the same problem  @DELL-Cares can you please comment on the official dell position on sleep support in linux? if it doesn't work, I will not place an order

Judging by how they are dropping s3 sleep support in XPS13 series (9300 has a buggy deep sleep, while 9310 doesn't seem to have it altogether), I would expect it is not supported in the newer versions of XPS15 as well...

I can confirm that deep sleep (s3) is not working with XPS 15 9500 with Ubuntu 21.04.
You can force it by setting "deep" on "/sys/power/mem_sleep", but the Laptop freezes after turning it back on.

I had many times, where the "s2idle" (modern standby) does not work and put the Laptop in my bag.
It overheated and I could not even touch the device, because it was that hot.
Had to open it and make sure, that the battery was still okay.
If I had not noticed that the Laptop was still running with closed lid, it might have caught fire in the bag over night.

I also had the known problem with the wobbly touchpad and had to fix that with some tape.

Oh and by the way, I am a software developer and also hate, that they changed the keyboard.
POS1 and END keys moved from FN+LEFT/RIGHT to the function keys on top. Always have to look at the keyboard for those buttons and can't type blind.

I have owned Dell laptops for 20 years and I have the feeling, that the quality and service is getting worse by every model.

All in all, don't buy Dell laptops any more!