The Economist's 'Big Mac' index is based on the theory of purchasing-power parity (PPP), which says that in the long run exchange rates should move towards the rate that would equalise the prices of an identical basket of goods and services (in this case, a burger) in any two countries.
At the very least its a good learning tool, and I've seen similar applied to Apple products which are also ubiquitous around the globe.
The Economist's updated FX valuations piece is ungated, so do check it out.
The biggest issue I see with this is now I'm hungry.
