Chameleon gives you the potential to do 840damage/180s = 4.7 DPS more. This is about ~5% of your damage output. This is not a straight increase in your damage, you will need to see if you are reaching the enmity of the tank first (Do not use the first ~20 seconds of the fight to judge this).

If you are well geared, then this skill is essential to further increasing your damage. Otherwise it is just a safety net.