Assassins are great.
You will notice in the tanking tree there is an ability in the high 30's that grants a flat 20% increase in armor. If you try to tank before that, you will suck. Do not attempt to level as a tank.
Deception is all about using Maul. Like literally, Maul, Maul, Maul. If you do not use a tanking companion, you will suck.
Madness is all about using shock. If shock is off cooldown, DO NOT THRASH. Save the force for shock. I use a healing companion when I spec this way because you also DO NOT USE MAUL.
The reason many people THINK the Assassin sucks is because it is one of the most complicated classes in the game, and if you do not realize how to use their abilities in relation to your companions/spec, they will be highly ineffectual.
In this case, don't hate the game, hate the player.