These cameras allow the user to take photographs and automatically develop the photos in mere seconds!

The photos are instantly produced.
If a picture went wrong, you can know it at once and can retake it.

Usually, such cameras are inflexible with lenses.
Photos produced are often found to be poorer quality.
Film is more costly, even without any developing costs.
No negatives (although some do now), thus reprinting may be a problem.

Probably, the best use of these cameras is for checking how a lighting situation or contrast and so on, will affect the real photo to be taken on a better quality camera. Not recommended for amateur photographers or normal users/occasions.