I really like problem solving and estimation questions during interviews, and I favor them over coding skills, etc. A question that I frequently use is how long would it take to evacuate a city like Phoenix, if there was a 24 hour warning that some dictator du jour is going to drop a nuke. It boils down to assessing a throughput of various modes of transportation: cars, buses, trains, airplanes, and yes walking. One candidate responded "by helicopters" at which point the interview was pretty much over.