Guavas are super high in vitamin C. You can eat them skin and all. I wait till they are fully colored to eat them. They get sweeter when fully colored, but here you will also be eating lots of fruit fly maggots in the fully colored ones, so people tend to eat them slightly hard and a bit more tart, but with less wiggles. I agree with Red Durian and like the yellow ones a bit more than the red, but both have an amazingly good taste. If you add them to any smoothie it gives the drink a very tropical taste. If the seeds bother you Behl you can always use a screen and remove them.
Here guavas are also used for making simple furniture, frames for sweat lodges, as BBQ wood, and many other small projects. Makes great charcoal also.