Smurfs have Calgar and Guilliman, plus a few other neat unique characters.
They can shoot after retreating from combat, making them great against speedy melee armies. Also they have +1 Ld, so they basically never run away.
Their stratagem is nice. 1CP to reroll 1s to hit for an infantry unit, or all failed hits if its a tac squad or an intercessor squad using it. You can drop plasma inceptors behind enemy line, and have them be as reliable as if they had a captain babysitting them.
The victrix guard detachment is shit though, use the generic Indomitus crusaders one for dakka intercessors.