Based on the commercials, it seems to solve trivial problems like answering trivia questions. Why can't it vacuum my room like a Roomba, fix my leaky ceiling or fix my power outage? That's what I call an useful home assistant. Being able to parse and understand my voice is nice but it's relatively useless if it can't perform extremely valuable tasks.
The things I'd really like an assistant for around the house--and which I pay people to help me with in various cases--are vacuuming, cutting the lawn, dusting, doing (and putting away) laundry, doing (and putting away) dishes, etc. Activating lights with a voice command is pretty far down the list.
That said, given enough smarts and interfaces to enough online services, a purely virtual assistant would still be pretty useful for a lot of people. "Book a trip with these general parameters." It uses my preferred booking services, knows my preferences, and comes back with some choices. Get to the level of at least a competent personal admin and that's a service I'd be willing to pay for even if it can't pick up a broom.
Expedia gave an interesting demo at AWS re:Invent around a digital travel agent. It was a very simplified demo--and, of course, Expedia specific--but I think it did give you a glimpse of what's possible.
Snark aside, once you get it paired with reasonable smarthome products, there's an element of magic there. "Alexa, turn on the kitchen lights" feels downright futuristic.
I know it seems a bit overt, but I'm pretty serious... I find myself reaching for google now more often than I ever have before. They're a step past Alexa when it comes to the sheer data that Google has to pull from. My only complaint is sometimes I hit clear all, and I can't get back to the reminders auto-set via email invites.
I also don't like that they've effectively deprecated the hangouts UX... they made interacting via SMS outside the phone pretty painful. I know there's hard costs to the stuff that came out of Grand Central / Google Voice, but I feel like when they first launched Hangouts, the experience was better overall.
In general using Google's services works so much better than anything else I've tried. That said, it's kind of creepy ay times.
Neither it nor amazon seem to understand the concept of a family or being in public even at the very lowest level. I can't imagine tying it to my personal account unless I lived alone. Anyone can use it to do anything they want. I have been rickrolled, all someone needs to do to rickroll an echo owner is walk by an open window and yell "Alexa (pause) play never gonna give you up by rick astley" then run like hell. It was funny the first time. I would imagine if I were dumb enough to link my amazon purchasing account to it, I expect I would now own a lifetime supply of dragon dildos from jokers walking by my window. Being able to respond to any human voice with no training sounds like an awesome idea, and it is, mostly, but unlike star trek daydreams, Alexa has no idea who's talking to her and if they should be allowed to talk to her, so she can only be given access to stuff that mostly doesn't matter.
IoT means closed non-interoperable silos but there exists a java project that emulates a Philips lightbulb Hue controller accurately enough to fool Alexa, and it can execute arbitrary URLs, and misterhouse automation can most certainly control lights and appliances using arbitrary static URLs so I have a pretty decent voice activated home automation. This brings up the point that most people cannot handle setting that up, but we're in the equivalent of the 1980s home computer boom where everyone has a different idea what a home computer would do, but we all agree everyone needs one. Alexa can do all kinds of weird things, surely it does something useful for everyone. Supposedly she has lots of sportsball features, none of which would interest me, but someone probably wants them. Everyone probably wants something Alexa does.
Being located in the kitchen I use the alarms and timers a lot while cooking.
Somewhat predictably, the best working use cases are ones that drive additional revenue for Amazon. Playing music from your Amazon Prime Music subscription, ordering laundry detergent and dog food from Prime Now, etc.
An excellent question especially in the context of adding another always on audio device to a private space. I think for many people the value has to be more than just a novelty to justify it.