Stormvermin are useful if Rat Ogre are too expensive for your army. With Halberds they have a Strength of 5, necessary to harm your opponent effectively. For that reason I think that Clanrats aren't very useful. They have only one attack and a Strength of 3. Gutter Runners are quiet useful if you are playing on a huge battlefield.
Assassins are too expensive for what they can.
With WS 5 and S 4 it is almost impossible to damage or kill a character model. Giant Rats are as cheap as slaves and can be fielded the same way. Their advantage is, that they can overwhelm your opponent. They can lap around the enemy unit regardless of winning or loosing the combat. If you are fighting against knights of any kind, you should take some Globadiers .