So after playing a pure Stone Sorcerer for a bit, I'm not really sold on Mirror Image. It gets held up as a godly gish spell due to the lack of Concentration requirement, but it has a really big flaw:

You lose an image when targeted by an attack (assuming you "make" the roll to divert the attack), not when hit. Given a STR based character is likely to have an 8 or 10 DEX, your images have an AC of 9 or 10, so they're almost certain to get hit every time.

For example, an enemy rolls 12 to hit you. They miss against your 18 Full Plate AC, but you have to make a roll for your image. You roll 8, so the attack is redirected to an image and causes it to poof. You're now down to 2 images even though you would have avoided that attack anyways.

A second level spell slot to avoid three attacks that you probably would have avoided anyways due to high AC seems questionable at best. Heck, I'd rather use the slot on another Shield when out of first level slots. Am I overlooking some errata or something? Wording as written it doesn't appear you can "choose" to fail the redirect roll in order to dodge the attack yourself.