Implanon has been in over here in Australia for some time now. It is actually an implant they place in your arm that continually regulates your hormones. I have not heard of many people who've had it put in but the ones I have heard about did not like it.
I've also heard that a few people have become pregnant on it, though I can't be sure about that.
They may have refined the product since I last heard anything about it (which would have been about 12-18 months ago) so it may have improved.
As for the disadvantages I found this article on the net:
Disadvantages
Some women may experience side effects such as:
- irregular bleeding patterns
A small number of women have reported side effects such as mood swings, abdominal pain, painful periods and hair loss. 
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