Bar Ends

The whys and hows of bar ends
Advantages:

Opens up the upper body to improve breathing.

Improve leverage on climbing.

Helps save your hands/shifters when crashing.

Looks cool.

L shaped bar ends can shield your hands from hitting branches.

More hand positions.

Disadvantages:

Added weight.

Short ones can catch the branches.

If improperly installed bar ends can damage you bars.

Moves all the equipment mounted on the bar closer in.

Bar end might be inserted into the rider's body as the result of a crash.

Positioning:

There is no exact formula for this. If you have never used bar ends, try them at 45 degrees first. If you find your hands are too far forward, move it up, if it's too far up, move it down.

Everybody uses a different angle, from flat to vertical. Most people use them at an angle between 45 degrees to 5 degrees up when measured from ground. Remember that all these things are very personal, there is no point in mimicking someone else just because you think that's the way to do it.