Claw Mastery functions like the barbarians weapon masteries by providing a passive bonus to all attacks made by the Assassin if she wields a claw weapon. The skill is great to have in that case, though whether or not to spend 20 points into it is another matter. Possibly there are other skills worth more investing in.
Many Assassins however do not utilize claw weapons at all. If that's the case, then this skill is completely useless and should only be taken as a prerequisite.