Long life, if used in a suitable environment. Saving future maintenance & cost.
Dramatically higher efficiency & lower power usage compared to incandescent lights (up to 80% cheaper to run). Good LEDs can produce 100 lumens of light per watts of electricity. Halogens are more around 10 to 15 lumens per watt. Here is a list of the most efficient LEDs.
Lower heat output, conventional incandescent lights waste about 90% of their power on heat output.
Typically lower infra-red and ultraviolet output. This can also mean a lower fire risk, especially compared to halogen downlights
Typically faster switching time and less likely to be damaged by on off cycles
Faster warm up time (especially compared to fluorescent lights). However some LED drivers / transformers do have a short startup time of around 1 second
A direct retrofit solution is now available for almost all types of light fixtures
They are usually more rugged. There is normally no glass to break and no filament to damage via vibration.
They are more environmentally friendly. They last longer, don't result in the emissions of as much CO2 and normally don't contain any toxic materials, like the mercury vapor in CFL.
Some LEDs can be setup to dynamically change colour, but this is not the norm for residential lighting.
Some of them run cool enough to install under roof insulation, so there is less heat loss from breaking the insulation layer.
Expensive to initially purchase globes, but prices have been falling year on year
Not all LED bulbs are dimmable
Possible compatibility problems with existing dimmers when retrofitting. The larger brand name manufacturers do fairly extensive testing, but you might not be so lucky with generic LEDs
Possible compatibility problems with existing low voltage transformers when retrofitting MR16 style globes
High powered LED downlights can be taller than halogen lights, which can be a problem if roof space is limited
May fail prematurely in high temperature conditions
Quality / colour of the light isn't always as good as incandescent or halogen lights, but is getting better. Light quality is measured using the CRI value. Here is a list of high CRI LEDs
Light output and colour quality can degrade over the life of the bulb (but other lights can also suffer this problem)
The need for a heatsink means some LEDs don't like being installed on their side as the flutes in the heatsink won't function efficiently.
They are a newer more complex product than an incandescent light, getting accurate specifications and advice can be difficult. Performance depends on good engineering, not over driving the chip, having a good heat sink and a stable power supply
The power supplies aren't standardised in LED kits. So in the case of a failure the globe and the power supply needs to be replaced.
There have been some problems reported of LED lights flickering. There are many causes for this. Dimmer incompatibility, transformer incompatibility and the ripple signals sent to control off peak electricity are the main ones. Many LEDs also flicker at high frequencies by design. There is no reason for a well designed LED light to flicker however.