O/T Compact Fluorescent Bulb cost effectiveness today:
I've discussed this before, but I thought I'd bring it up again since the prices of CFB's have dropped.
Currently you can find a 60-watt equivalent CF bulb for around $1.33 or less. With 4.6 times less energy use and about 4-times the life of a typical incandescent bulb, they have about 18-times greater efficiency per unit. However, when purchase cost is factored in they are only 2.76 times more efficient per unit purchased.
A typical incandescent bulb lasts about 2,000 hours. The Compact Fluorescent will start paying dividends over an incandescent bulb at about 2900 hours in this example.
If you have a light in your house on for an hour every day, it would take you nearly 8-years to break even on the compact fluorescent purchase. Since the average family moves every 7-years (sooner if you had a mortgage in 2005-2007), you're likely buying lights for the next people who will inhabit your household.
From my perspective, it pays to replace burned out incandescent bulbs with CF bulbs only if the light is going to be lit for 1.5 to 2-hours a day on average. Otherwise, it's likely you're not going to get a return on the investment.
I did this today because I was thinking of replacing a light in my bathroom with a CF bulb, but I calculated the payback would be 32 years for that CF bulb, so it made me think twice about doing it. Furthermore, the numbers are even worse if you need to be able to use a dimmer switch.
It may sound trivial, but I've got something like 200 light bulbs in and on my house, so it adds up.
