Good if you bother planning your apartment around it so it is actually capable of doing its work - meaning you need to think of reducing amount of obstacles it has.
There is also a big difference in quality, I would not buy the cheapest shit.
Once you have it, just like a dishwasher it becomes very handy, you let it do most of the work and then sometimes clean the areas yourself that it was not able to, meaning you need to vaccuum less yourself.
Imagine a square room that gets a set amount of dust build-up each day. You press a button, go outside for a walk while the robot removes the dust from most of the areas. At the end of the week you mop up the areas it missed.