To me, right now it excels as an interface when you are busy doing something else. Ours is used most in the kitchen and living room. Timers, streaming music/audio and basic facts all work great. Home automation is improving and will get there as systems learn to play nice. That seems like a lot for $50, but maybe that's just me...
The killer app hasn't been created yet (and possibly not even envisioned). I can't wait until they are connected and I can issue a command to move streaming from one device to another one in another room ("Alexa, pause for Jim", "Alexa, continue for Jim").
I think the biggest downside of voice UI right now is a lack of authentication. Someone outside the window could yell "Alexa, open the door".