The legal replacement of 100 W incandescent 'normal' light bulbs with energy efficient bulbs (CFL, LED and Halogens) started on the 1st September 2009. From then on incandescent light bulbs continued to be phased out with finally the 60 W golf-ball shaped and candle shaped bulbs being made illegal to sell in retailers from January 2011.
These energy saving light bulbs do tend to have a warm up time but otherwise to that they are a practical and efficient replacement to the incandescent bulbs.
Energy efficient bulbs use approximately 80% less energy then the standard bulbs and last about 10 times longer. Each new bulb installed will save on average £3 a year with an average saving per household of £45 a year.
With the legal directives and also from a common sense perspective, replacing your incandescent bulbs as quickly as possible is common sense.
More information on the topic above can be found on the Energy Saving Trust website which provides more detail on these measures.