Alexa works well on a smartphone. It was able to quickly answer a question about the weather and read me my daily news briefing. I even turned off my living room lights from work.
Still, it's not as good as Siri or Google Now for several reasons.
First, you can't just call up Alexa from across the room as you might with Google Now or Siri on newer iPhones. You have to manually launch the app. Also, while Siri and Google provide visual cues, like graphics that show the latest sports scores or the forecast, Amazon only provides answers through your phone's speakers. It would be better served by adding some eye candy.
Amazon is going to want to get this in as many places as possible, especially for folks who are used to having an Echo in the living room or Amazon Fire TV on their televisions. For folks who already use Siri or Google Assistant, however, Amazon's going to need to spice this up a lot more.
Maybe that's why it's making its debut on a phone that's probably only known among enthusiast circles.