Okay, got my book, here's my take on the "Ghost step".
-The invisibility lasts 1 round.
-It is supernatural, so the invisibility does not dispel if you attack
-It uses one Ki power per round of combat.
Ninja only gain 1 Ki power every 2 levels, and one for each point of Wisdom modifier. So a Ninja's Ki is not great enough for it to remain invisibile for a great deal of time.
Example: at 8th level, a Ninja with an 20 wisdom could stay invisible for a total of 9 rounds, a wizard of the same level, with an intelligence of 14, could cast greater invisibility and be invisible for 80 rounds.
An invisible ninja could theoretically sudden strike 9 people during that time, dealing +4d6 Sudden strike damage to each, for an average of 108 damage, +str, and choice of weapon.
An invisible Wizard could drop 3 fireballs (@ 8d6 each (save for half)) and an Ice Storm (@ 5d6 (no save)), both of which are area effect spells and have the potential to damage multiple foes... and still have 2nd level and 1st level spell slots open for acid arows and magic missiles, and he'd STILL have time to run away from the scene of the carnage.
Really, Ninjas are the best argument I've ever seen for a rogue pursuing the Spring attack feat chain. Start 15 feet away, go invisible, Hit and move.