It definitely will reduce the amount of electricity used however it's not a linear relationship between light output and power consumption since the efficiency of the light source will in most cases decrease at lower output.
So reducing the light output by 50% will save power, but the saving will be less than the 50% reduction in light output particularly if the lamps being dimmed are incandescent (including halogen).
For incandescent or LED lamps, dimming will increase lifespan that's pretty straightforward.
For fluorescent lamps, dimming is inherently somewhat problematic and how it affects lifespan is an "it depends" question.
For halogen lamps, in theory dimming will actually decrease lamp life. Consumer grade halogen lamps, particularly 240V ones, are however always something of a gamble in terms of lifespan and the halogen cycle isn't necessarily of any value to start with such that dimming may either increase or decrease lamp life in practice.