I just upgraded from the Nexus 6 to the Pixel XL, and I have no complaints about the sound quality. I do miss the 6 inch screen, but otherwise the XL is a definite improvement, particularly battery life and the camera (admittedly the N6 camera was horrible). It also has a very nice fit and finish, comparable to the iPhone.
That said, I’m on Project Fi so I didn’t have many options. The next Pixel will supposedly lack a 3.5mm jack and cost a lot more, whereas the current model is on sale, so it made sense to get it while I could.