So I have had my S6 for almost a week and here are my responses to your questions:
The thing I am most surprised about is its longevity and how dependable the battery is. After working out the Cellular Standby drain issue I got 4hrs 45minutes of SOT with 14 hours off the charger and extensive camera usage. For my usage and history with devices it is almost unheard of. My Note 4 was marginally better.
I am also surprised with the consistent performance of this phone. Recents is always fast, apps are always quick, the camera always opens fast... I could go on.
The thing I dislike the most, this will probably be fixed, are weird issues like Pocket (the app) force closes automatically on opening for me, but it works for others. I had some lockup issues in Chrome while browsing video intensive sites. Im sure they are just small hiccups, but honestly im just nitpicking because there is really little to be upset about with this phone.
I would absolutely recommend this phone to a friend. I have been an AndroidJunky for over 5 years now and I have sold 10 times as many friends on iPhones instead of Android based ones. Typically you give up something with Android. You could give up build quality, dependability (crashes and stuff), OEM software update commitment, camera and other things in comparison to the iPhone, so when an non-tech head wanted a recommendation I would almost always push them the way of the iPhone. That changes with the S6. The phone is consistently good, from the camera to the call quality to the look and feel of the OS.
One more thing that surprised me about the S6 was the new "Simple Sharing" feature. You can share pictures with anyone with a phone number, full resolution shots. A text message gets sent to them with the prompt to enter a pin that is the last 4 of their phone number. No more highly compressed crap with MMS.