My feeling is that they are more trouble than they are worth. They are basically just a float valve like in a toilet and if you have a leak in the pool they will continue to fill it and often the first indication there is a problem is that your water bill has gone thorugh the roof. They do require constant attention and adjustments. My feeling is that when you water is low refill manually and if it gets low a lot check for leaks with a bucket test.
In some parts of the country they are standard equipment on builds (and might be somewhat useful in very arrid, desert areas). In other areas they are very rare but it really boils down to your own decision in the end. Another thing to consider is whether the pool will be heated because the autofill will be adding unheated and possbily cold water to the pool. If running on the garden hose to top off the pool is just too much work for you then an autofill might be useful. (Then again, if just topping off the pool is too much work then perhaps you should reconsider pool ownership! It's really one of the easier pool maintenance tasks!