4 out of 5 rating for Serpent's Skull Adventure Path
First, I've only read this adventure path, not actually played it. With the exception of the first and last parts, this is a story of jungle exploration. It could have worked just as well in a horror, pulp, or steampunk game, and uses some conventions from those genres. Read like this it is an excellent story.
The first part begins with a shipwreck leaving the players stranded for a bit of survival training. It is a good standalone, but actually quite separate from the plot. The players do have a chance to make connections and enemies that become important later. Parts 2 to 4 detail finding and exploring an ancient city deep in the jungle. The final two chapters are about discovering the caverns under the city and are less true to the exploration genre and more normal dungeons. I'd consider skipping at least part six, or possibly combine parts five and six into one shorter adventure, but I was never one for long dungeon slogs. Others might love this part and want to skip parts 1-4 as pure introduction.