I use compact fluorescent lights for about half the lights in my house. I started doing this living in an apartment where I needed many lamps and didn't like the heat produced by standard incandescent bulbs. Being the engineer that I am I did a cost benefit calculation before buying that first bulb. Times have changed and I wanted to run through the calculations again.
Let us assume we have a lamp that is on 5 hours a day. If we use a 60-watt incandescent bulb it will use 60x5x364/1000 = 109.5 kilowatt-hours of electricity. I pay a little over $0.09 a kilowatt-hour for electricity, so that works out to be $9.86 a year to run that lamp with a regular bulb.
Doing the same thing with a 13 watt bulb gives us 13x5x364/1000 = 23.66 kilowatt-hours of electricity and $2.13 to get the same light. Now a 13 watt compact fluorescent costs about $2 these days and should last for 5 years. But even if it did only last a year, in economic terms it would save you 9.86 - (2 + 2.13) = $5.73 per year (or $7.33 per year if you assume it does last 5 years). Depending on how many lights replaced this can add up to significant savings. Plus it keeps rooms with large numbers of lights cooler and more comfortable. And of course there are the environmental benefits of using the compact fluorescent light.