I don't think this game will be like Fallout. In Fallout, when you kill all the raiders in a particular building, then you've cleared that area for good. Enemies never seem to respawn.
In Rage however, it was said in an interview that when you clear an area of one type of bandit, either another type of bandit respawns later, or maybe even mutants. So, since all types of bandits and mutants have their own fighting style, you'll get a whole new experience when you re-enter a previously cleared area. I just hope it isn't like Borderlands. In Borderlands, you better not stay around exploring for too long after clearing an area of enemies, because they'll respawn before you even leave the area. Right before your very eyes, every single enemy respawns. I can't tell you how frustrated I was every time that happened. I definately couldn't explore like I wanted to. At least let me leave and come back before respawning every single enemy.