LED fixtures don't need bulb changes. They are sealed units. They are rated, as Jason said earlier, at about 50,000 hours of life, white or color. At 12 hours of use per day, every day, they'll last about 11 and a half years. Halogen or incandescent bulbs would get swapped ever couple years along with it's lens gasket, at best. Color changing Halogen fixtures will also need a new color wheel in that time frame. Halogen fixtures usually have more than one bulb too.
While a 500 watt, white, incandescent fixture will put out more lumens per light fixture, the Halogen and LED fixtures are fairly close to each other, comparable to a roughly 250 to 300 watt incandescent fixture. The 500 watt can light a 20x40 pool but it will take 2 LEDs.
LEDs use between 50 and 90 watts of power but put out as much light as a 200 to 300 watt incandescent, depending on the manufacturer. Lets take the power usage, bulb and gasket and color wheel costs into account and the payback isn't that long considering the price. If's even faster if you don't change the bulb yourself.
There are a lot of positives with LED fixtures.
Scott