The Assassin is essentially a souped-up Thief.  The bonus to hit humanoid advantage is nice, but only when you're having to fight humanoid critters.  However, the Assassin does have good weapon skills.  With Dodging showing up as only a Minor skill and severe armor restrictions, the would-be Assassin would be well advised to dump level bonuses into END and work on being VERY sneaky.  The limited magic ability of the Assassin almost forces reliance on potions, but if access to the Mages Guild can be had (hint: buy a "Bouyancy" spell and practice it until you're sick of it), development of Illusion skill can make the Assassin equal to a Nightblade in short order.