LEDS will not burn out like tradtional lights because unlike other lights LEDS are Diodes and they have a much longer life to them in fact most LEDS will slowly start to fail over a few thousand hours. Fluorescent tubes on the other hand can just die.
's are rated for 35,000 - 50,000 hours of life compared to Fluorescent tubes at something like 10,000-15,000 hours.
Cost is to run them is lower, the size is great, they are shock resistance since they are solid, nothing to break in them.
Yes they cost a lot more to start with but once installed the cost will be much cheaper over the years since they will last easily 3 times as long as other bulbs will.
Problems with LEDs are that instead of something like a compact fluorescent is that instead of just a ballast, connection and blubs they need drivers, resisters, heat sinks bulbs... They need to have a little bit more going on behind the scenes to get them going which ups the cost.
Only reason I know about them is I work with signs, used to be most people used standard bulbs to light there signs, then neon and most recently LEDs. We installed a sign about 8 years ago now for a cafe here in the city and we havent been back to service the LED
's once they still give off the same light. Some Neon signs we put up last year have been serviced 4 or 5 times already for fading, failure, and indoor signs using fluorescents well those seem to get serviced every 2-3months.
Then again tho this is all my experience from Sign installation and repair not from aquarium lighting. Personally if I could I would get LED
's in a heart beat.