I "own" a Nest Protect alarm. I liked it and was planning on outfitting the rest of the house with them during renovations this summer. I will not be buying any more Nest products/services now. The risk is too high to have them in my home.
I got given one for Christmas. It appears to work as advertised. But it also slightly spooks me that it is an Internet-connected black box with a microphone, and the kind of thing that under the UK's proposed Investigatory Powers Bill the government could probably insist be hacked to spy on me. (The microphone is used as part of the self-test routine, to check the thing is sounding properly).
Replace them with Z-Wave or Zigbee products instead. There are a lot of hubs out there which don't require cloud connectivity or a monthly connection.
INSTEON does a solid job. Their protocol is proprietary, but the devices aren't cloud dependent. You can either use a cloud hub device, or just a serial adapter to a PC or third party controller. They have no ability to take away my Insteon thermostat's functionality, or mine data from it. Because all translation of it's communication protocol goes through my computer, and is interpreted by open source software.
I would stay away from anything Zigbee. It has become a complex mess