They've been screening Man of Steel since March so I'm not really sure why there aren't more reviews. But then again a lot of critics just got to see it this past weekend.
I've noticed that usually the Wednesday before a big release all of the reviews start pouring in. So I bet it hits 100 reviews today.
As far as "This is the End" they know that they have a legitimate alternative to Superman and a hit on their hands. So their really pushing this movie with early screenings, ads and word of mouth is good.