It's basically the latest fad diet.
Wouldn't it only be a 'fad' diet if it didn't work? However, it should work, since it restricts processed food consumption, meaning that the person following the diet shouldn't eat junk food. And if we don't eat junk food, we stay away from unhealthy sugars and fats, consequently losing weight...unless of course you warrant 'fad' by implying that we aren't getting all the nutrients we need from the diet, and hence, may become malnourished.
However, I do agree with the idea that paleo diet seems temporary -- kind of like the 'gluten-free' fashion we had going on, until nutritionists unravelled that foods lacking gluten were instead substantiated with other fats and sugars. I suppose we just have to wait for health-food experts to unearth some sort of innocuous evidence against the paleo diet, such as the being thrown back into the primitive times because it involves consuming 'cave-man' food.
I think it's best to just stick with the health food pyramid and its 5 basic food groups.