They do a "passable" job. My front lawn is about 15 acrres, it has some plantings and three fountains and a reflecting pool, they vavigate around all these obstructions and with the random walk pattern seem to cover most of the area, it takes them a good 6 days to completely cover the lawn, The lawn looks well kept and it frees the gardner up to do those things my wife wants him to do. The gardner only has to spend about 20 minutes hitting the spots that the robots miss (usually the back ends of spots...)
We are probably trying to push these little guys past what they were supposd ot do, but so far they have run for two months non-stop (other than battery recharges), no failures of any kind.
They are also safe, I have two children and my brother and sister (same ages as my children) are always outside playing, and these things have yet to clip a toe off....
There was a time there where the dogs wouold turn them turtle, but they lost the fascination with doing that.
If you are interested I will throw one out on some uncut grass of varying height and let you kow how it does. Just give me the word.