Kydex is an extremely durable acrylic/PVC alloy, created and distributed from the Kleerdex Corporation.  Kydex is extruded in a wide range of colors, patterns, textures, thickness and grades to optimize long-term performance, appearance, and cost effectiveness. No other thermoplastic stands up to impact, scratches, and gouging like Kydex. It offers notched Izod Impact resistance to 18 ft. lbs./in. (953 J/m) and is able to withstand various levels of abuse. Due to its high degree of toughness and resilience, Kydex can also be stretched at room temperature to twice its original dimension without breaking.

Kydex holsters are more concealable.
The material strength of Kydex allows for greater strength with a fraction of the thickness associated with leather concealment holsters. A thinner holster significantly decreases printing. 

Kydex holsters have no break-in period.
Your holster will fit the same every time.  Whether it is your first draw or your 1000^th draw.  This means that there are few; if any, adjustments needed to start carrying your holster and firearm.

Kydex holsters are weather-proof.
Kydex holsters can be worn in the rain and do not swell or contract due to sweat, humidity, or extreme weather.  In fact, Kydex will help keep the moisture produced by sweating from coming into contact with your firearm because Kydex will not absorb moisture, unlike most conventional holsters.

Kydex holsters are easier to clean and keep clean.
If your holster becomes dirty it can be washed with soap and water. Detergents may be applied to it.  Gun cleaning chemicals or lubricants will not affect the Kydex material.  

Kydex holsters will not collapse when you draw your gun.
When you draw your gun with a Kydex holster, the mouth will stay open and the shape will not change. This allows you to use one hand while re-holstering your firearm. 

Kydex holsters are less expensive.
There are some very specialized and well crafted Kydex holsters that seem pricey, but as a generalization Kydex holsters are less expensive to purchase.

Kydex holsters are fast on the draw when comparing them with conventional holsters.
In most IPSC / IDPA competitions, the fastest competitors are usually wearing Kydex.

Kydex holsters will eventually wear on your firearms finish.       
On a positive note, sinse the only retention contact location on the holsters is around the trigger guard, you will not have the same type of barrel wear that you get with conventional leather or nylon holsters.