I have an S6, my first android in several years. It is by far the worst phone that I have ever had. After a couple days of usage you must manually clear the cache to get the phone working again. This phone has major memory usage issues. 10 - 15 seconds between pressing a button and getting an action is almost normal for this phone.
I wish Samsung would fix their current phone and not brush its problems off by moving to a new model.
MicroSD or replaceable battery are not the biggest turn down on Samsung's phones.
The software, Samsung software is the issue.
I had enough with Samsung stock OTAs on my previous SGS3, Note 3, SGS 5 and SGS 4 mini.
Samsung's stock OTAs are simply completely un-optimized, un-stable, fully loaded with unremoveable bloadwares, slower as time goes by, buggy, etc.
I have the impression that Samsung only makes half-finished ROMs.
Of course you can fix those with custom ROMs, but how many understood even what root access means. (yes, I had to run custom ROMs on my Samsung phones, otherwise, they are almost completely useless only after 1 year)
I moved away from Samsung to Sony and 1+, the software quality is a lot better by those 2. It is like night and day.
My Z2, Z2 tablet, 1+1, my sisters Z4 and my aunt's Z3 are all excellent, even without root, even after 1 year (only on 1+1, Z2 and Z2 tablet, the Z3 and Z4 are not yet 1 year old at home).