Blink is a power that's useful for everything. Moving around, blinking up behind guards, and just scaring the crap out of attackers. For my non-lethal playthrough, a combination of Blink, Bend Time and Possession got me through most situations without a hitch. In my lethal playthrough, I learned the value of rats. Devouring Swarm is crazy powerful, distracting up to 6 different guards, allowing me to slice them to ribbons. 3 or less guards I don't even need to touch, they'll just get eaten on their own.
Almost every power is overpowered in it's own way, really (when upgraded, at least). Blink makes you completely uncatchable and assassination becomes really easy. Possession allows for instant escapes and infiltration without any risk. Darkvision lets you see EVERYTHING, avoid all traps and become so rich you can buy every upgrade available before taking on the Lord Regent. Bend Time basically kills everyone in the room and can be used as invisibility, too. Devouring Swarm really just turns any group of enemies into a all you can eat buffet. The only power I can't abuse completely is Windblast. Powerful, but rather situational and combat is already a confusing blur of swords.