It is economically viable. Iceland gets most of their energy this way. There are a number of operating plants out in California.
The short comings are you either have to build in a geologically unstable region such as the fault line system, or volcanically active areas where the energy is readily available, or spend a great deal of money to drill deep enough to find sufficient heat.
When you look at the long term cost of coal and natural gas there are rather large areas of the US where this geothermal energy can compete. OTOH in areas like the Great Lakes and much of the Great Planes, that heat is a very long way down. There are on-line maps showing these depths nation wide.http://www.bing.com/images/search?q=depth+of+geothermal+energ