Had a live case for my Nexus 6. Aside from the NFC button being pretty much useless (had to put so much force in when pressing that it felt like I was going to break my phone), the case itself was great. Didn't add much bulk to the phone and was form-fitted nicely.
For my Pixel XL, I have the OEM clear case. I like it. There have been plenty of people complaining about it due to the sharp edges and thinness of it. But I enjoy both of those qualities. Give it a nice feel without adding bulk.
I currently have a live case in transit for my wife's Pixel so I'll provide an update if needed about its quality when it arrives next week.