Benefits
Lighter periods, which in a third of cases will stop alltogether (amenorrhea).
Fewer family planning appointments - women with an IUS only need to see a doctor once a year for a quick check up.
The localised release of the progestin (or progestogen) into the uterus results in less systemic side effects compared to Progesterone only pill or Depo Provera.

Will not interfere with sex.
Unlike the pill, it can be forgotten (apart from needing a replacement after 5 years).
After the IUS is removed, normal fertility returns quickly.
Over 99% effectiveness.

Disadvantages
During fitting:

perforation of the uterus or cervix during insertion.
sepsis, though risk may be reduced by proper aseptic technique during insertion.
For up to three months after fitting:

Periods can become irregular
irregular bleeding
Headaches.
Breast tenderness (mastalgia)
However, these side effects should be temporary and disappear within three months.

Rare but serious adverse reactions include:

ectopic pregnancy.
intrauterine pregnancy (small risk miscarriage and bleeding, but likewise if attempted removal).
Pelvic inflammatory disease - usually associated with sexually-transmitted infections.
embedment of the device in the myometrium.
ovarian cysts (usually not dangerous, and rarely require treatment).
breast cancer.
Hence, the device is contraindicated in patients who may be at increased risk for these reactions. More information regarding warnings, contraindications, and other adverse reactions may be found at the manufacturer's website.