Properly designed LEDs are wonderful for places that are hard to reach, since they last so much longer than halogen or CFL bulbs. How long they actually will last is hard to tell exactly, since in theory it will take tens of thousands of hours to reach the end of their life (or longer). Because of this, lighting companies are forced to test the bulbs using acceleration techniques to simulate years of life in a short amount of time.
LED bulbs last longer because they don’t have the same issues that standard bulbs do. They simply don’t have the parts (filaments, igniters, etc) that other bulbs do that break and cause a bulb to burn out. Since there are no super fragile parts and there is no breakdown that naturally occurs, they keep working for years and years.
In fact, when most companies list and average life for an LED, they refer to a time when the light produced by the bulb will diminish to 70% of the original maximum brightness. The bulb itself will continue to work well beyond that time frame.