They make great devices and S Voice seemed like it would be handy if you didn't care for GN, but I did my best to keep from relying on any of their proprietary stuff.
I switched from S Health to Google Fit months before I got my 6P, because I knew I was switching manufacturers with my next device either way.
I may be labeled as a Google fanboy, but I'd much rather keep my data, stats, etc. with them and know that it'll quickly and easily transfer to whatever device I get next. Before the 6P, I was thinking Moto X, so I would've lost it all there as well.
Same goes for my contacts. I had a Moto phone a while ago that wouldn't remember that I wanted to save contacts to my Google account, so it saved to a Moto account or the phone or something a few times and I ended up losing some contacts.
These manufacturers are good at doing stuff that kinda locks you in to only their devices. I get why from a business standpoint, but as a consumer who likes the choices we have when looking for new devices, I don't want to be severely limited because all of my info is stuck with one OEM.