I'm loving the Mate 9 - I came from an S7 Edge. The only thing I miss is the safety net of the IPxx rating (not that I ever used it), other than that the Mate 9 is the better phone for me. The screen is amazing, it might be a lower resolution on paper but in practice I can't really tell the difference and the extra size is really appreciated. I was never a fan of the curved screen as it just feels like wasted space and made the phone hard to grip. The Mate 9 screen feels gargantuan by comparison and that's a good thing in my book. The sound quality from the internal speakers is leaps and bounds better than the S7, not sure how they've managed to squeeze such a great sound out of such tiny speakers but I continue to be impressed by them.
Battery life is way better on the Mate 9, now that I've got over the initial 'new phone syndrome' I am able to come home from work with 40% battery left which was just unthinkable with the S7 - it would usually be on its last legs!
Someone mentioned about avoiding the Mate if you want to use Samsung Smartwatches? All I can say is I've not found that to be the case at all... I've got a Gear S3 that I was using with my S7 previously and it works just the same on my Mate 9. I can even download new watch faces and apps from the Samsung store on the Mate 9 and sync my fitness data to the S Health app on there too.
The fact it comes with a 3 year warranty and 3 month accidental damage cover is just the icing on the cake!