Had my S8+ for 4 months now, in beta so its on the last beta, and it's running so smoothly no complaints about speed here it's super fast. Finger print sensor took me a few days to get used to and now it's muscle memory. My wife has an iPhone and I picked it the other day and tried to put my finger on the back right to unlock without thinking (I have a slim case on it which I think helps with finger placement too)
The only things I can think of off the top of my head to improve is the speaker and let you remap bixby button or remove it, nothing else leaps out as needing any massive improvement. Screen is amazing, the battery life on the + is great, everything runs fast, even samsung's software is pretty good, though I use Nova Launcher myself.