First of let me say I think the S6 is going in a great direction and the new RAM and CPU is going to be great.
That said I'm a little disappointed in the direction the new case is taking. While I love that they are adding a trackpad to the keyboard I wish they kept the same case design as the S4. Having had a Surface Pro 4 with its kickstand its just not useful for when the device is sitting on your lap. It doesn't really sit there comfortably or stable while typing or using the trackpad. Whereas the current setup the S4 has worked really well when it's sitting on your lap. I'm hoping that maybe they will later down the road release a similar S4 type design or that a 3rd party makes one for the S6 that has that design.
The only other complaint is the new SPen. I'm a huge fan of the SPen for the fact that it never has to be charged. I don't even have to think did I remember to charge my SPen? The need to remember to charge a stylus is really what turned me off of the iPad Pros as its just not something that I could ever remember to do. I'm a little concerned with the early hands-on reviews that say if the SPen gets jiggled just a little it stops its charging on the back of the S6. That said if it's like the other Samsung products with a SPen you should be able to use the other not charging ones on it with no issue.
All in all, these are not big gripes just things I will miss about the earlier Tab S4 model.
This is exactly what bugs me about the Tab S6. It was so convenient that you didn't have to charge the S pen. The additional Bluetooth dependent functionality of the S6 pen isn't worth the trouble either. It is bad design that the Air Command button does not show up when charging the S pen (the Air Command shortcuts are not dependenton Bluetooth) . The Air Command button with its associated shortcuts is essential for me and I want access all the time when I work on the tablet.
I have no personal experience with the keyboard cover because it is still not available in my place (4 weeks after the tablet was released here...). In Internet discussions people complain about the way in which the back part of the keyboard cover is attached to the tablet: Samsung uses a suction strip that doesn't hold up when dust settles on it. There is also a serious concern that it will deteriorate pretty soon when the cover is removed often.
Otherwise the tablet is an excellent device and fun to use.