Having had the S8 for a couple of days, so far so good. I've come from a Note 4 running Rooted Android 7.1.1, and the S8 so far is as slick and responsive as that was (unlike Touchwiz on the Note 4 which was sloooooow). There are so many features on the S8 its a bit mind boggling, e.g. waterproofing, facial/iris/fingerprint unlocking, Always on, wireless charing etc blah blah the list is long.
What impresses me most of all about the S8 is the sheer amount of effort and changes that have been made since the S7. The new Samsung skin/interface and apps for example, whether you like them or not, are so different from the old versions. Its funny to watch Apple roll out incremental little changes to iOS year after year, but they are definitely lagging behind now - they recently ordered many millions of new OLED screens from ... Samsung.