Just a little food for thought based on my first experiences with Kor'sarro Khan and why I think he is an excellent addition to a mechanised Space Marine army.
Chapter Tactics
No doubt the most obvious reason to take Khan, your army exchanges Combat Tactics for Outflank. Most people take advantage of this by outflanking their Bike Squads or Assault Squads to get to grips with the enemy faster and without having to slog across the table. Outflank in general is seen mostly as a means to get your assault units in close range more reliably. However, if your opponent castles up in the centre, or deploys in a refused flank, you risk your units arriving piece meal with no further benefit, or on the wrong side of the table, respectively.