Assassin III is not a game-changer like radiance, but it just makes so much sense. It allows you to vorpal, DPS, stat damage, and debuff all at the same time, as well as free up two inventory slots.

If you're going to get Assassin II, there's no reason not to go ahead and grab III.

On that note, Assassinate (from Assassin II) is some of the most fun I've had in this game, regardless of whether I'm soloing slayer areas or zerging with the guild. With practice, you can pull off assassinations in almost any situation without too much difficulty.