It's a good counterpart to the Rogue's Uncanny Dodge, with enough downsides that I don't think it'll make them into tanks. It has a higher minimum than Uncanny Dodge, able to negate smaller hits entirely, but doesn't scale up against big hits. It only works on physical damage until 13th Level, which might or might not be a big deal depending on how the Monster Manual is styled now. And it only works on one hit a round, so it's not as good at holding the front line against multiple foes as a Barbarian, say.
All in all, a very solid defensive upgrade to a notoriously squishy class, but the utility will rather depend on the DM's encounter design style.