Lag for me. Most noticeable is the Home screen where the elements are all redrawn which causes a 1-2 second lag sometimes. Not all the time, though. Going to and from the Flipboard App on the left screen is pretty bad sometimes with a stall. I know a lot of people uninstalled it.
Twitter has lag, especially in the Twitter browser, so does Facebook and so does Chrome. Not all the time but enough to notice. It's definitely not as fluid an experience like I was expecting. Certainly, nothing in the league of the iPhone 6 Plus which is smooth as can be and probably spoiled me to some extent.
That said, the hardware in the S6 is spectacular so I'm expecting that Samsung/Google can fix this in a software update, hopefully soon.