I bought a DELL XPS 15 9550 and a DELL Thunderbolt dock TB15 240W.
Unfortunately, they are not working as expected: from time to time, in a non-regular way, but frequently, they disconnect. More precisely, I use the dock to have a 4K monitor, internet through cable, a printer, a USB hard disk, a keyboard and a mouse. Sometimes, only the monitor keeps connected while the other devices disconnect from the notebook. I only get them connected again if I restart the notebook and the dock (i.e. unplug from electricity). I have updated all drivers, etc but the problem persists. It is quite dissapointing!
In addition, the fan of the notebook start immediately when it is connected to the TB15. It seems that this connection (Dell Thunderbolt cable) is heating something inside the notebook. Very dissapointing!