Advantages:
You only use it when it is needed.
This method is readily reversible. The moment you want to become pregnant, you can stop using it.
Can be inserted hours before you have sex, so it won’t interrupt lovemaking.
Properly fitted it’s comfortable and barely noticed.

Disadvantages:
Reliability depends on how carefully it’s used. If the girl knows what she is doing it has a reliability of 95%. But in practise 18 out of every 100 women who use it become pregnant after a year. So it’s not very reliable.
Every time you have sex you must remember to use it.
You cannot remove it after sex: it must remain in place for hours.
Must be fitted by a competent clinician. This must be repeated after you gain of loose weight (more than 6 pounds) and after delivery.
Can cause bladder infection in some girls when it presses against the urethra.