Minto said:
the variable resistor in the dimmer just dissipates any of the power that isn't used in the light bulb as heat and sound (the slight buzzing you hear from them)
No, thats not how they work at all (well not these days, old theatre dimmers from days gone by did), if they worked like that then they'd get a lot hotter than they do, and be a lot bigger than they are
They work by phase cutting... switch on at the beginning of the the half cycle and bright light, switch on towards the end and dimmer, everything resets at the end of the half cycle to start again
Also, it should be noted that adding series resistance would reduce the power taken anyway, just not in a very efficient way becuase of the power that remains, some goes into the load and some into the series resistance
Anyway to answer the OPs question... yes they do save money (at least assuming a domestic meter that measures true power, rather than apparent power, whether they save money on a meter that measures apparent power is a question that its two late to thing about, though its irrelevant to this thread), but not as much as you imagine, if you dim the lights to 50% you are using about 75% of full power. If you are using many downlights, split them off into separate banks and switch them individually (oh, side point, derate dimmers by 50% when using with halogen lamps if thats what you are doing... the transisants are greater than with normal lamps
)