I've had the phone for a few days now and can give a little insight from a real world point of view. The UI is smooth..is it perfect? No. But definitely the smoothest version if TW out right now. I have experienced very minimal lag..and the only stutter is I did see were when multiple apps (like 10) were running. Soon as I closed them it was back to smooth sailing. Any phone can lag if you trying to make it. In real life who's opening 20 apps in 5 minutes going back and forth. And let me say this..this "lag" is only an issue for us phone nerds in forums like this, mostly because we are looking for it. Normal users will not even notice it.
Now the battery..I will admit..its not great. I'm getting 12-13 hours with moderate use. Bluetooth, Wifi and GPS only on when needed. Web browsing, FB, Instagram, texting and watching a movie on my 40 minute commute to work. It also tends to get warm quickly. Again for most users it will be just fine. I told my wife I was only getting 12 hours of battery life and she said "whats wrong with that? sounds good to me". Though she averages slightly better on her S5.
Overall im enjoying the phone so far and I think most people will too. Is it for everyone? Absolutely not. Im actually considering the Note 4 because im one of the few people that prefer the faux leather back of the Note to the all glass of the S6. Seems more durable and the S6 and Note are close enough in specs it wouldnt feel like that much of a downgrade but I'll decide that in the next few days. The S6 is beautiful but very slippery, you definitely need some type of case which kind of defeats the purpose of getting such a beautiful phone.
All in all I agree with most of the reviews. Great design, great camera, smooth (not perfect) UI. but the battery is average