Speakers with decent sound (sure, your separates system is going to sound better, but pretty decent). They have some really fancy wireless mesh tech in an era where that's unheard of. They pick up your local music collection and have a controller application on most OS's that allows you to build stereo pairs or tv surround systems wirelessly, arbitrarily group together rooms in your house, stream music from wherever to wherever and generally just have music in the places you want it with little hassle. I still have some Sonos play:1 speakers I got in 2012 that are going strong, even though they live mostly outdoors now.
Now, if those aren't your use-cases, and you really do care about high fidelity sound (I don't), then it's probably not for you. But it has been great for me and I've built up quite a lot of speakers.
Unfortunately (and here's the subject of TFA) - in the middle of last year they screwed the pooch, and half the time their app doesn't even load any more :/