Every “Diet” or “Nutrition Plan”, whatever you want to call it, has positive and negatives. Though I think we should eat more grains than The Paleo Diet would say, it is a very sound diet style to follow.
As for the grain comment, just because our “caveman” ancestors didn’t ride around on horses, doesn’t mean that when we finally did it was bad for us. If anything it helped propel human civilization. Same thought with grains, just because they didn’t consume a lot of whole grains doesn’t mean that whole grains are horrible for us. Some research might point to the contrary given the nutrients and fiber that they give us.
But once again, the Paleo Diet ideas are a very good start to any nutritional diet.