Some were too quick to jump on and a subset of those were too quick to jump off.
For someone who is a 25 year old rookie, I think 400 plate appearances is a reasonable sample size to draw inferences.
After 405 plate appearances, he is slashing .239/.318/.330. His defense is above average, which has brought his WAR up to 0.7. Extrapolate that to a full season and he is a 1.0-1.5 WAR player.
Relative to my expectations, he's been a slightly positive surprise.
His splits hint that a platoon might be a good way to use him. His slash line against lefties is .209/.264/254. His slash line against righties is .247/.329/.347. A Peterson/Castro platoon at second might be in the Braves future.