We just installed Windows 10 upgrade on my Dell XPS 1645 and it went fine except for some niggles here and there. Everything works out of box but if your Dell XPS 1645 came with Bluetooth onboard, you will have some trouble getting it to work. So I located some Windows 10 Broadcom Bluetooth drivers from the Lenovo web site which seems to have worked. While the installation program didn’t complete but drivers did get installed as seen in the device manager.
Also if you use internet via Ethernet port, there is a frequent drop in connection over wired LAN in windows10 on xps 1645. This is a well documented and known issue. Touchpad works but only basic features. Special gestures like pinch zoom, dual finger scroll etc are not working. So I do miss two finger scrolling.
It is important to note that you must first upgrade from Windows 7 to Windows 10 and then do a clean install to get avoid as much of the legacy un-optimized junk as possible. This will also ensure that you are correctly register the Windows 10 license.