I just went through this process - here is my experience.
First off, I have multiple Apple devices (iPhone's, iPad's, kids have iPod touch's) so I just want to put that out there. The Apple EcoSystem is important for my family, but I'm a tech geek so I do not discriminate and I'm not so locked into the EcoSystem that I cannot switch to an Android phone or tablet. HOWEVER, I do want products that work and are easy to pick up and use. I use a tablet mainly for a media consumption device. Reading, Twitter, YouTube, web browsing, casual games etc.
I recently bought an Air 2 and loved it, but prefer the smaller tablet (i.e. Mini, Galaxy Tab S 8.4 etc) so I gave it to wife and decided to buy the GT S 8.4 to give it a try. This wasn't my first Samsung Tablet or for that matter my first Android tablet as I've had the Nexus 7 2.0. Here are my honest impressions of the Galaxy Tab S 8.4 after a little over a week of intensive use.
Positives:
-AWESOME screen. Bright, colorful . . . the Super Amoled is the best I've ever seen on a tablet. If you only care about the display then absolutely check out the GT.
-It is very thin and light. Easy to hold to read a book etc.
-Multi window is cool and useful. Watching YouTube video and doing some google searches split screen is nice.
-MicroSD expansion is nice to have.
-Google Now is really, really nice.
-Obviously customization of Android is a positive.
Negatives (I realize given how many Apple Devices I have some of these are subjective in nature, BUT, I'm trying to be as device and OS objective as possible) :
-Battery life was not good for me. Whenever I checked what was running etc. the beautiful display (set at about 30% brightness) was constantly the cause of over 65% of my battery usage. I barely got through a day most times without having to plug it in (Spotify, web browsing, YouTube etc).
-For $399 it didn't feel in the least premium to me. Rubbery, dimpled back still feels a bit low end (I understand this is very subjective and was NOT a deal breaker for me).
-Touchwiz is still too bloaty for me even though it's gotten much better. I did download and install the Google Launcher which made things better but specifically on the Samsung it just isn't a cohesive experience and it shows. I understand that some like this and want multiple app stores etc, but it just isn't for me.
-Also, Android is coming along nicely, but again in my humble opinion it's still way behind iOS in smoothness, polish etc.
-The device hangs and stutters way too often. For example, when I would touch the YouTube App it would hang for a couple seconds before it opened. I know it's only a couple of seconds but it shouldn't happen on a high end tablet.
-Chrome was slow and laggy. Also, scrolling stuttered constantly. Double tapping to zoom into text and read magazines etc was unreliable and spotty.
-Didn't love the aspect ration of the screen. Likely something I would get used to but I like the wideness of the iPad Mini much more for reading, browsing etc.
-Some apps are just not as polished or don't work as well. For example, I got Rayman Fiesta Run and it suffered so much slowdown it was unplayable. Say what you will, but it takes away from the experience when this happens on a supposed high end, high spec tablet.
-The Fingerprint Sensor is a joke compared to TouchID. Sorry, if someone wants to argue this they just haven't tried or extensively used TouchID. I believe that TouchID does not get enough credit for how well it's implemented and how accurate it works. TouchID was revolutionary for me (yes I said revolutionary) as I need to have my devices locked for security purposes (work and kid proof) and while it's admirable Samsung tried to add the sensor it is just way too unreliable to be used consistently as the unlock method.
-Holding the device landscape mode created many problems for me because of the capacitive buttons on the bottom of the GT. Too many times my palms or fingers would hit those buttons just simply from re-adjusting grip or trying to access the mult-tasking bar that is accessed from swiping to the left from the right side of the screen.
I think that's it - sorry for the long post. To sum it all up, many of these things might seem small individually, but the sum of the parts created a less than optimal experience for me given the $399 price tag of the Galaxy Tab. Therefore, I returned the GT got the iPad Mini 3 instead (Yes, Touch ID is worth the extra $$$ for me).
One last note - I did purchase the Samsung branded wrap case for the GT as well ($60) and it was atrocious. If you get the GT, get a cheaper one on Amazon.
Good luck.