When I bought my Roomba in 2013, it cost as much total as I pay my cleaning ladies to come once every two weeks. If your floors get dirty easily, it's not really going to get them spotless, but it'll get them far cleaner than they'd otherwise be.
You just schedule it and forget it. As everyone says it doesn't do as good of a job as you do but the main benifit is it's consistent about doing that job more frequently.
I have a dog and need to vacuum at least once a day, currently.
Without a robot vacuum, Id go crazy.
You save the 20 minutes once a week.
That's it. That is the whole point. A slight convenience. I use one in a 1 bedroom apartment.