I have the Pebble watch, married to my recently purchased S4. Previously it was paried with my HTC Desire (Bravo) with Cyanogenmod.
The initial setup was a bit stranger than the HTC, but eventually worked.
Now with the pebble notification enabled in the "Accessabilty, Services" section, the stupid "Talkback" option seems to be on, and speak "Webpage Loaded" and stuff with the default browser. Even with toggling it on and off, the stupid thing just keeps speaking. Using Chrome browser stops the voicing, but it's annoying as hell.
Either way, I don't notice any noticable battery drop in my daily use. I am up at 4:15am and disconnect the charger, go to the gym then work, then gym again. By the time i put the phone back on charge at about 8:30pm, i have about 18% battery left. That's fine by me.
If you go to your gallery the top bar won't hide either. Its a bug in TouchWiz and Samsung hasn't fixed it since the S3. Oh and I doubt you eye tracking works either.