“It’s one of the most sold-out things that I see on Amazon,” Mr. Wingo said. “It’s an unusual thing for Amazon to be out of something, but for the Echo, it’s usually in stock just for a couple weeks before it goes out of stock for a few days — so it feels like they’re having trouble making enough of the devices.” Mr. Wingo noted the Echo, which Amazon sells for $180 only through its own site, was selling for $200 to $300 on eBay.

When the Echo was introduced in a goofy video in late 2014, on the heels of the failure of the Fire Phone, it was widely ridiculed. The Echo’s utility was not obvious, and in its earliest incarnation, it seemed a bit of a ditz. But there are a couple of reasons it has earned such raves from users.

Image Credit... Stuart Goldenberg

First, it’s simple to learn, and its voice-recognition capabilities are more intuitive than those of many other vocal assistants (like Apple’s Siri or Google Now). More than that, it keeps gaining new powers.

During an interview at a media event last week, Dave Limp, Amazon’s senior vice president for devices, said the company created the Echo because it had seen interesting possibilities arising out of advances in microphone technology, speech recognition and cloud connectivity. Amazon’s engineers spent years perfecting the device’s unusual capabilities. Unlike competing assistants, the Echo can be activated hands-free from far across the room (Siri only works from a couple of feet away), and it can decipher your voice in noisy environments, even when it’s playing music.

Amazon also worked to make sure the device responded very quickly. “Early on in the product, to play music took eight or nine seconds, and it’s just unusable when it’s like that,” Mr. Limp said. “Now it’s often 1,000 milliseconds or 1,200 milliseconds.”

The speed makes a crucial difference. Compared with the trudge of chatting with Siri, speaking to Alexa feels natural, closer to speaking to a human than a machine — and even when it gets your request wrong, which in your early days with the device will happen often, it doesn’t feel like you have paid a huge penalty for trying.