For $4, you get a lot for your value.
King: The addition of the King and Robber make Carcassone maps more complete and satisfying without upsetting the point system. Instead of long roads and large cities sitting unfinished, all players will compete to finish projects. This way everyone benefits, but of course, someone will benefit more. Like Traders and Builders, there is a reward for finishing other projects. This continues this same logic.
You also get some extra pieces. The 'bridge' piece which allows two seperate cities to cross over each other is wonderful for ending (or starting) fueds on shared cities. The other pieces fit some necessary combinations not in other sets.
Scout: Hunters & Gatherers is solid stand-alone but this expansions adds variety. Your new pieces will give you special abilities, some seemingly more powerful than others, but it all depends on how you play them. You'll get a renewed zest for the game by adding these pieces. Just these five pieces add a world of complexity, making each game different.