Ravingdork
Explorer
It's not too powerful. It is useful.
It is less useful in campaigns where combat does not occur on a daily basis.
From my experience, it hardly ever works. It blocks a successful attack about one time in three (unless the DM somehow lets the player know how effective the NPC attack is and also the roll on the D20).
So one combat day in three, it helps. Even at this limited utility level, it is more helpful than Feather Fall. But, Expeditious Retreat could be helpful one combat day in one with a good player.
It's been my experience that most GMs say something to the effect of "Does a 23 hit your AC?" In such a situation, it is easily a powerful utility spell.