Scott Hulen randshomes.con
About as useful as the average foot size. Forgive me for my bluntness, but this statistic is often misused and misunderstood. If you are comparing similar sized, age and quality homes in close proximity it can be a factor to consider, but otherwise it doesn't mean too much.
I once sold a luxury home with a view for over $1,200,000 for a lower price per square foot then another home I sold 3 blocks away that was a 1550 square foot starter home.
If you need to know the value of a home to buy or sell there are no shortcuts. Zillow, Eppraisal and others can give you a rough idea, but actual valuation comes from a detailed market analysis done by an agent who knows the homes involved.