Well look at the title... I really love the fire breath shout, but I have the feeling it's rather weak compared to other shouts... Lesser minions die with just Yol (first level) but the stronger opponents hardly get damaged even if I fully charge the shout. I even have meditated on it with Paarthurnax and it stays weak -.-'' Same for you or am I just using it wrong?

Depends on the enemy you're firing, some enemies are immune or resistant to fire damage.