Treasure Maps
There are five types of treasure maps.
Treasure Map, Laboratory Map, Space Map, Spooky Map, and Underwater Map

The only map that I think is worth buying is the laboratory map. It gives you unlimited access to the Secret Laboratory's Lab Ray. The lab ray can strenghthen your pet, weaken, it give it a level up or level down, it can also change your pets gender. Sounds good right? Well, there is one catch, you can't control the lab ray! It is all random. The only thing you can do is fire the ray, and hope your JubJub comes out a JubJub. But you should just be glad if your pet comes out alive.

I would just sell the rest of the map pieces.
Space map gives you about 10,000NP, 3-4 items, and a level up for one pet
Treasure map gives you about 15,000NP, and 3-4 items
Spooky map gives you 5-8 spooky foods, and about 10,000NP
And the Underwater map gives you a petpet and some NP.
The only map that is worth it is the lab map!

Well, anyways, I put all the pieces together and made a picture of what all the maps look like when you are done! Use these pictures to check which pieces you have so you don't end up buying more than one of the same piece.
Space Map Treasure Map
Spooky Map Underwater Map
Laboratory Map