Bixby seems to greatly outperform Google Assistant and Siri.
Bixby is much more helpful than Siri on function-based commands. It can "take a selfie," giving you a countdown before it shoots. Google's Assistant, built into the Pixel and other Android phones, is a bit more capable than Siri, but still lacks Bixby's ability to tie together actions. Only Bixby can follow, "Open my last photo and post it to Facebook."
Using (mostly) ordinary English, Bixby can operate every last one of the S8's gazillion settings, as well as all the functions in 12 built-in apps such as the Gallery, Messages and internet Browser. Another 21 apps including Email, Gmail, Maps, Yelp and Uber are listed as experimental.
You and Bixby can even have your own secret code, called "quick commands." I taught mine to "activate deep throat"--launch the voice-recorder app and take a memo.
Bixby struggles in the same areas as the other AIs. People need to be able to give clear directions.
Bixby's bigger challenge is grokking what you really mean. When I opened a photo and said "brighten this," it turned up screen brightness instead of adjusting the photo.
"Play Lady Gaga," you said? Sadly, Bixby heard "play lady kaka"...and then replied, "Here's the time." Yes, that really happened, one of the many times Bixby just couldn't get me. "Caller" became "collar"; then there's "super California lipstick expealidocious."
Bixby's voice could also use lessons in elocution: It pronounced 2:48 "t'forty eight" and read an error message as, "Looks like there's a bit of problem" [sic]. Samsung says updates it rolled out even in the last day improve these problems--but clearly it doesn't have Google, Apple or Amazon's years of voice experience.