If you're planning on using leds for moonlights youll need quite a few of them. What i did was get 12v leds out of an auto section and wire them up to a 110 convertor. They look/work good but i need more.
For light fixtures the advantage is that they use less power and last a lot longer than other bulbs. That's part of the reason people have been making LED lighting for reef aquariums, plus they don't get as hot as metal halides, but they're EXPENSIVE, which is why not many people have tried them out.
I haven't really been able to find any plain old light fixtures just for viewing fish that were LED so I'm not sure of the cost, though I have seen a few people build their own.
As far as moon lights, there's really no advantage, esp. in freshwater aquariums, other than being able to view your fish at night.
I don't know of any particular down sides of LED lighting other than initial cost.
I know for a fact that a company have started manufacturing LED in fluorescent housing. It runs about $150 for each bulb. Its 10K lifetime rating so I would say its not bad. Its going to be cheaper in the future of course.
Let me check my PopSci and see which company it is... or you could do a google search.
For moonlights etc LEDs are good, you dont need high light output, their efficiency is pretty good, and they last for years.
Although it's possible to replace normal lights with LEDs, you need a LOT of them, and high intensity LEDs are still expensive. To replace a large metal halide light fitting would cost $thousands.
No doubt the price will come down in the future as the LEDs are mass produced, but for now they are really for low intensity specialised applications.
Also in reality even the best LEDs are not as efficient as flouro lamps (getting close). The reason they run cooler is that they are drawing much less power.