I've had similar experience with my ASUS N76V: it worked fine for a number of recent releases, but only with the upgrade to Ubuntu 15.10 did the touchpad start working decently (I couldn't turn on two-finger scroll before, because that resulted in phantom multi-finger gestures).

The current situation with XPS 15 seems much worse: http://ubuntuforums.org/showthread.php?t=2301071

For decent experience, one should wait until April until 16.04 comes out. And some issues will probably remain for a release or two after that.