Friends have one which does a great job - trundles around day and night (it's got a little headlamp), and goes to bed when it rains.
Their lawn is immaculate (think croquet lawn)
We thought about it, but we have a very lumpy grass area (moles, voles and rabbits)
The main reason we did not take it seriously, was I think it would get stolen within a month.
We live in a remote rural area, but everything has to be locked up.