I figured I'd make a guide related to XM-Ls and tint, binning, CRI, and all the other details I can think of.
There are 3 basic tints.
Cool White is a very pure white and is what most lights are. This generally has a low CRI (color rendition index) and will not show colors that well. Some people say that it is to blue. It is generally around 6000k.
Neutral White is considered to be one of the better tints as it still seems fairly white, but does have a higher CRI. It tends however to be slightly tan colored rather than a pure white. It is generally around 5000k
Warm White is on the extreme end, it is very yellow like the color of a incandescent bulb. It has the highest CRI so it is great for outside but inside it can get annoying when white wall hunting. It is generally around 3000k.
The tint is defined by a number followed by a letter, an example of this would be 3C. If the number is a 0, 1, or 2 then it is most likely cool white. If the number is a 3, or 4 then it is most likely a neutral white. If the number is from 5 to 8 then it is most likely a warm white. The reason I say most likely is because I am not specifying the letter.
Note: Going off the the Cree datasheets 5 is NW, but I find it to be much wamer than neutral.
To determine what the tint is like one can refer to this image.
To get an idea of what different kelvin ratings are like one can refer to this chart which shows them more clearly than the above one.
For XM-Ls the brightness bins range from S4 (which is the least efficient) to U3 which is the most efficient. The order is S4, S5, S6, T2, T3, T4, T5, T6, U2, then U3. Most lights are a T6 bin unless otherwise specified.
In between bins there is an approximate 12% difference in brightness but depending on the efficiency of the LEDs it can be as little as 0% and as much as 26%. These are both extreme examples based on the fact that a bin can vary as much as 7% and there is a 12% average difference between different bins.
Part of the reason that there is a variation in efficiency is due to the amount of phosphor on top of the LED die. The purpose of the phosphor is to convert the blue light produced by the LED to a more white or yellow light. Naturally as the amount of phosphor increases less light passes through it. This is why a warm white LED is going to be a lower bin than a cool white.
Below someone asked the question of why a blue LED does not produce as much light as a white LED even though theoretically there should be more light. After some research on the topic I found out that this is because the eye is very sensitive to white or yellow light, but very insensitive to blue light.
So sadly, we will most likely never be able to get a 1000 lumen 10 watt blue LED.
Recently, (December 2012) cree released the XM-L2, which is approximately 20% more efficient than the original XM-L. This means that a XM-L2 T6 is approximately as efficient as a XM-L U3. Unless it is specified, the light does not contain a XM-L2.
Interpreting The Specs of a LED
So now we know the basics of how a LED is rated now. So let's say you see XM-L T3 7B for sale. Now you know that will be less efficient, warm white, and an XM-L.
CRI stands for Color Rendering Index. The CRI of sunlight is 100, the CRI of our average CW LED is closer to 65. NW is approximately 75 and some WWs get to 80. Naturally you want as high of a CRI rating as possible so it is easier to distinguish between different colors.
A high CRI does not always mean it has a low kelvin rating, but it generally does. Nichia makes great Hi-CRI (high CRI) LEDs. Currently the most powerful one they make is the Nichia 219. It is slightly less powerful then a XP-G, is NW (5000k), and has a CRI rating of 92.
Links to XM-Ls-Based on Tint
Locating a XM-L with the tint you want can often be hard so I am going to paste links to a few different ones here.
XM-L 0D: 1,
XM-L U2 2B: 1,
XM-L2 T6 3C: 1,
XM-L 5B: 1,
XM-L 7C: 1,