Well I don't have the S6 - so that's helpful info as I try and determine what my next phone is going to be next month
Don't get me wrong. I absolutely Love the phone. There's not a single phone out there with a screen that rivals it. The Camera is absolutely a pleasure to use (Even the selfie camera is great). the performance of app's and games is stunning.
The problem is, despite all the hardware chops, Touchwiz still just gets in the way of itself. The memory management is buggy and you'll find things reloading themselves that just should never have left memory. iPhone users complain about having to constantly have Safari reload webpages, Well, Same thing happens in Chrome on the S6.
There are UI frame drops, more noticable when waking the phone and it smooths out, but its still noticable.
Battery life is, inconsistent. Some days I can get 5 hours on screen time and 15 hours between charges, then somdays i'm out of juice after 8 hours and 3 hours onscreen time.
Sometimes Apps take a few seconds to load. Especially if its an app that i had already opened recently.
once a week, I have to reboot the phone. Something is causing some form of memory leak. its noticable when I press the homescreen button, and my homescreen + widgets take up to 5 seconds to draw.
After a reboot, some default Apps are reset: for example, your default music player is always samsung player. So, for example, I reboot my phone, unless I conciously start Play music, my blutooth connects and Samsung player starts, Every time.
As i've said, i've used Stock Android as well on other devices. And Had NONE of these issues on anything running stock, or Cyanogen. My 2 year old nexus 5 running 5.1.1 just felt smoother overall.
Which sucks, Because the hardware for the S6 is gorgeous. the in hand feel is fantastic, and the actual performance when Touchwiz isn't getting in the way is fantastic.
Which is going to only backfire IMHO on this promotion. Samsung still is going to have work to be done to convince iPhone users to switch. IPhone users already have a great screen, great camera. the main things people notice. So software issues and glitches, that don't immediately make things more enjoyable than using iOS isn't going to win people over.