Personally having played an assasin on layo, the only redeeming feature of the class is the fact that I can use the poison I make with no chance of failure in application. Its the one thing that sets an assasin aside from an ordinary rogue as far as I am concerned. So, rogues spend their time in larceny and skullduggery but not specifically learning to master the use of poison. Creation of poisons is not really a terribly difficult task but the care and application of such is highly specialised, hence here is where the assasin comes in. A Rogue class that has spent time and care developing the skills required to not only make but to work specifically with poisons.If ordinary rogue classes, or any class for that matter had a good chance of using poison then I might as well kiss my character goodbye because its the only thing in my mind (apart from RP reasons) that gives you any incentive to make one!
Except for the impressive ability to paralyze your targets with Death Attack. Obviously it takes time to develop, and the expense of carrying intelligence boosting items can be substantial, but it is a very useful ability.