I tried the S6 and have had it for about a short week now, I will most likely be returning it due to several reasons but I'll give you my experience with it.
It's really well built and has a excellent screen and screen size. I'm not really into phablets, although I had a stint with the 6 Plus and enjoyed it, I still ended up trading it back for the regular 6. This phone just seems like it has the perfect screen size and comfy ergonomics.
I'm not a huge camera user, but the camera on this phone is outstanding and very easy to use. I think the picture quality on it is just a little better then the iPhone.
I really enjoy the fast charging. My battery demands aren't very high, but it's nice if I'm ever in a pinch and I top it off really fast, though most of my charging habits are just at night when I go to bed, I leave it plugged in.
Here are the negatives...
The S6 still lags. Yes I'm sorry, many are going to disagree with me here, or maybe I'm just extra sensitive to lag and frame drops, but TouchWiz will still be TouchWiz here. Don't get me wrong, it's much, much faster then it was before.. but case and point, web browsing some of my favorite forums and message boards I frequent, the S6 jerked all down the page and stuttered at times. It would smooth out eventually but it would do this continuously. It takes a long time for the RAM to cache everything and this is just annoying. Just out of curiosity I turned on my iPhone that is hooked to Wifi, and to no surprise, browsing and navigating is just smooth as silk. Safari performance just crushes Chrome and it isn't even close here.
Touch ID on the iPhone is a little better. I use Touch ID without any problems, the S6 fingerprint is good as well and fast, but often times will miss my print and I have to redo it. I tried clearing them and redoing my prints about 3 times and still had the same result. It works fine, but about 8/10 times my print will fail and I have to do it again.
Battery life is good during actual usage, I was able to get just right at 5 hours of screen on time with lower brightness, but the standby time is awful. (I'm on Verizon so shouldn't be any standby drains). I went to bed with the device at 87% one night, and woke up and it was at 75% or so. I factory reset the device just to make sure it wasn't a bug and it still did the same thing. This is just nuts, my 6 would never drop that much on standby time, I haven't really been able to quite get that day and a half battery life on the S6 yet despite light usage because of its standby time. My iPhone 6 can hit two days of light usage.
The lack of iMessage has been a little annoying. I know this won't apply to some, or will with certain exceptions. Almost all of my regular contacts have iPhones and all of us running iMessage is much better, able to share pictures and video messages seamlessly. I don't send a lot of video but we do share pictures quite often and it's nice that I could load multiple pictures into a message and just throw it at my contact and iMessage will send it without a hunch.
I really wanted to like the S6 but these downsides will probably do it for me, especially the lag. I was really disappointed with this because I almost trusted Samsung to deliver a lag free experience. Even my Moto X 2014 overall feels more smooth and snappy. There are times where the S6 is incredibly smooth, but other times TouchWiz rears its ugly head and the phone just gets crippled down in stutters and frame drops on the screen, especially with web browsing. Also doing texting would lag to, if I was typing really fast or typing a long text message, the keyboard would start to lag a lot and slow down embarrassingly so.
The lag and standby times on the battery has mostly done it for me. It's such a shame because this is a really nice device. I can't excuse the 2k display for the lag, the polish on the iPhone really shows and makes me appreciate it a little more. So it will most likely be going back. This is greatly improved over the S5 but I guess TouchWiz will be TouchWiz. How will it hold up overtime with long term use? And then the topic of software updates.. we all know how those can be if you don't have a Nexus device.
It's probably back to the iPhone 6, but it had a good shot.