Well I got my S8+ a couple days ago, the move from the iPhone went fairly painlessly, mostly because I was already using google photos, calendar, contacts and music. Though the Samsung switch thing mucked up my text messages, they where all out of order and new texts arrived half way up list so had to delete all old messages.
As far as fluidity goes there is the odd stutter here and there but the phone is still very fast 95% of the time, it helps I am coming from a 6+ iPhone which was getting a little slow, though was still perfectly usable. Overall I am pretty happy so far and love the massive screen in a body the same size as my 6+ all for several hundred less pounds than a iPhone 8+ and nearly £400 less than the iPhone X at those price differences it's hard to fault it. Hopefully it holds up for a couple years without any issues.
I saw the report that Apple has lost market share in the UK (while gaining in the US) can't help but think that UK customers are finding the new iPhone prices just high enough to put them off, and combine that with Samsung's deals on the S8 and S8+ at the moment it's hard to justify the iPhone unless you are a die hard Apple fan or stuck in imessage for your social life.