I have been using them for quite a while, and have 3 in my house: 1 usable in the living room and kitchen, 1 in the bedroom, and 1 in the garage/workshop.
The more you have them, the more you'll use them. They keep adding a lot of new features. Recently, you can enable calling between people that have them. I gave my brother a set of 3 of the Dots, for less than $100, for Xmas, and all I have to do to call him is ask Alexa to do so. Don't have to pick up the phone.
Also useful for woodworking: Alexa, how many millimeters is 58 1/2" ? Lots of conversions available.
As far as listening all the time, it is listening for the "wake word", Alexa (it can be changed to two other words) and, if you want, you can go online and delete everything it has heard in the past.
Once you buy one and install it (requires an app on your mobile phone), Amazon will send you a weekly email with new things it does, and another good source is CNET.com. You can sign up for their weekly Alexa email.
Steve