Excellent first impressions here ... Safari is noticeably more responsive, and much faster in the visual elements, especially scrolling. This is something that really bugged me in Yosemite, very happy that this is fixed. Assuming this is the result of the new "Metal" graphics framework.
Find my friends in the notification area is cool. Split screen is very cool. Everything is just ... super cool so far.
Haven't noticed anything negative yet.
I like the idea of the new System Integrity Protection feature ... meaning: I can't screw up permissions on vital parts of the system, other apps can't screw up these permissions, and no more permissions repair option because it's not necessary anymore.