You want to eat them when they are just slightly squishy on the outside when gently squeezed. If hard, let ripen a few days. If it was picked too early, it never ripens right, and starts to turn brown inside.
Ideally, it will be a healthy dark green around the immediate inside of the firm skin, and a nice light green leading up to the large seed. It should be easy to cut through with a knife—if not, it's not fully ripe, but harmless to still eat.
It tastes good by itself, with a subtle flavor, but most use a hint of salt, lime, cilantro, lemon, or onion to set it off. I think its best with all of those minus the lemon, then mashed up into guacamole. However, it is really nice as untouched slices on hamburgers and other meats.
It is worth trying.
Cut em in half,remove the pit and stuff them with a seafood salad (shrimp,tuna), squeeze lemon over it.
DELICIOUS!