To me, DR 10/ magic against one attack and negating crit, sneak attack, and poison seems okey for a 1st level spell. By the time that 1st level slots are not so rare the dr /magic is easily overcome. After that, its how much do the other things come up in your campaign. The players I have played with get pretty discouraged when playing spell casters and wasting spell slots. I don't see it getting cast that much if there is a decent chance that you will waste the spell. Now, if you know you are going to get hit anyway, I.e. Your AC is so low as to be meaningless, then it may be a different story.
I've seen a balance consideration that goes something like this: is it so good that it will be used by most characters most of the time? Then it's too powerful. Is it so weak that it will never be used? Then it's to weak. If you require a player to waste a slot scribing or knowing the spell, then spend a spell slot for the day casting it, and it may or may not work, I'm just going to cast Mage Armor, Shield, or Ablative Barrier.