Most people are familiar with common power usage terms (i.e. Watts) that are used to describe the amount of energy consumed by an incandescent light bulb (i.e. 60w or 100w bulb). However, what is actually important when comparing a new technology like LEDs is the quantity of light being emitted because the amount of power drawn (Watts) can be dramatically different between two products that produce the same quantity of light. A common measure of light output is Lumens, and a standard comparison would be that a 60w tungsten (traditional) bulb would output around 600-650 Lumens (for more, see this Wikipedia definition).
So, how much energy can be saved? I said earlier that LED lights are much more efficient. One way to compare this is my creating a ratio of the two terms just described - Lumens and Watts - by asking the question "How many Lumens are output for each Watt of energy consumed to generate that light?" For example, the earlier bulb I mentioned consumed 60 Watts of energy and produced approximately 600 Lumens of output. This could be described as 600 Lumens from 60 Watts, or the more standard way to describe this would be 10 Lumens per 1 Watt or 10 lm/w. As a comparison, many LED lights generate upwards of 75-100 or more Lumens per Watt, so, as you can see, LEDs are approximately 7-10x more efficient than this common incandescent bulb. Further, the technology is improving, so this efficiency is increasing every year.
Second, LED lights can save you significant cost on maintenance and upkeep. This is a broad statement and the exact amount of savings will depend on how each individual decides to maintain his or her lights. However, some basic facts are true. For example, LED lights are rated to last as long as 100,000 hours as compared to 7,500 - 20,000 for many traditional lights. Because of this, they will need to be replaced approximately once for every 5-10 changes of the traditional lights. Each change has a cost associated with it that includes the light itself, but often additional hardware such as ballasts, as well as labor, which can be significant, particularly if special equipment such as a ladder truck are required.
Other ways that installing LED light technology can save you money include reducing recycle / disposal fees (because they last so much longer), potential for government and/or utility rebates, and fewer lights because of increased efficiency of lighting.
For examples of different installations, go to this gallery.