A light-emitting diode is a type of semiconductor that emits light when current runs through it. LED lamps/Lights are composed of numerous LEDs grouped and some other components together. Recent improvements in technology have improved the luminosity of LEDs and made LED lamps/lights a suitable replacement for incandescent or fluorescent lamps, HID.  

Pros:

• Long Life, if used in a suitable environment. Saving future maintenance & cost.
• Dramatically higher efficiency & lower power usage compared to incandescent lights. Good quality LEDs can produce 100 lumens of light per watts of electricity, even higher. Halogens are more around 10 to 15 lumens per watt. 
• Lower heat output,  to 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, some are the norm for residential lighting, like Hue, Lifx.
• Some of them run cool enough to install under roof insulation, so there is less heat loss from breaking the insulation layer.

Cons

• Expensive to initially purchase globes, but prices have been falling year on year
• Not all LED bulbs/ LED Light 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. 
• 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 EMC/FCC. Well designed lights shouldn't have this problem however.
• There have been some problems reported of flicker. 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.
• Glare Issue

In conclusion

LEDs are significantly more energy efficient, longer lasting and durable than conventional light bulbs. Therefore, the initial investment(Good Quality LED Lighting) is repaid in a short time. It is worth paying that little bit extra to make sure you get a well designed and manufactured LED light to guarantee the best performance.