I have a $20 hunk of plastic on my ceiling too. It beeps whenever I cook a bunch of things, won't stop until I get out the stepladder and literally press a button on it, and when it needs a new battery it helpfully lets me know by beeping just infrequently enough that I can't tell if I'm imagining the beep, or tell which smoke detector it's coming from.

The Nest seems like overkill to me, but let's not pretend that old-fashioned smoke detectors are some kind of awesomely perfected tech.