agree with krash. They are very usefull, but you need a lot of points that can be destroyed 'easy'
I love an assassin into a shades unit. It must be about 10 shades or they are easily destroyed by magic or shoots.
get the stars and manbane and you can beat a lot of heavy guys. I killed a unit of juggernaugts with commander in a row. (shades and assassin, lucky I guess). It's almost a 2+ hit and 2+ injury.
Another way is an assassin into a ground unit that you know si easily attacked. Maybe b.a.corsairs that are one of the hardest units against shoot, usually they are attacked by riders and you stand and shoot and then show the assassin. This one with rune of khaine+manbane+aditional hand weapon.
I'll try shadowblade in a 2K thematic combat this weekend. A classic light vs dark in a 4 vs 4 commanders

, I'll tell you how it goes.
As Krash said, more than 2 is too many points. Maybe against shooting armies are more weak, but against 'hard' units is required with manbane.