We recently replaced most of our incandescent bulbs with instant-on CF light-bulbs. I cheated by upgrading our light output (using 100 watt equivalent bulbs to replace 60 watt incandescent bulbs) but each bulb should still save 34 watts.
I loosely estimated our daily light usage (over weekdays and weekends) on the newly CF fixtures as the following:
Bedroom (1 bulb): 1 hour
Bathroom (3 bulbs): 2 hours
Hallway (2 bulbs): 6 hours
Sewing Room (2 bulbs): 1.5 hour
Living Room (5 bulbs): 7 hours
We are saving (57 bulb hours * 34 watts) 1938 watts a day, approximately 2 KWh a day, or 60 KWh a month, or $6 a month (at $0.10 a KWh). We should pay for the CF light bulbs in seven months.