Just bear the following things in mind:
They don't recommend staying on the injection for longer than 2 years because of bone density loss.
Women sometimes get spotting while on the injection, which is pretty irritating.
It can take up to a year for fertility to return on the injection. This may sound like a good thing, but the problem is, lets say you want to change to a different type of contraception. All of them will require you to wait for her period. And you have no idea when her period might come back - it can take months. So, if you want to stop using the injection, you will need to go back to condoms until her period comes back, and then you can switch.
I personally think the Gynefix IUD sounds like the best contraceptive available on the market. Google it. No hormones, no needing to remember anything, and a small increase in bleeding which usually goes away after a while. And lasts 5 years.