Halogen Replacemant bulbs are steadily getting brighter & more efficient.
The best of the bulbs we offer now compare favourably with a 40w halogen, but are not yet quite up to the brightness of a 50w halogen you may have in your kitchen or bathroom. 
In many situations this would not be a problem, but if you are trying to create similar light levels we would advise you fit up to half as many again LED bulbs as the halogens you are replacing.
For example, replace 6x 50 watt halogens with 9x warm white SMDs.
This would then consume 27 watts as opposed to 300 - saving 90% of the electricity.
Over a year, at 4 hours a day, this would save 365 x 4 x 273 = 398,580 watts or 399 Kw.
At 20p per kilowatt this is a saving of £79 a year.
9 SMD bulbs cost £198, so will pay back their investment in 2½ years.
Lastingt at least 20,000 hours, (over 13 years at 4 hours a day) they save 13 x £79 or £1027.
Even if your bulb only lasts the 5 years we guarantee them for your total savings
for this particular set-up would be 5 x £79 or £395. 
And the price of electricity is unlikely to fall or even stay the same...