Like someone said above, if you want kids in the next couple of years, don't go on it - it can sometimes seriously muck up your cycles and take ages for your fertility to return.
I had two shots of it when I was younger and it really didn't agree with me - was hormonal psychobiatch from hell for over six months basically, piled on weight, headaches, felt rotten and I hated it - was persuaded to have a second shot with the promise that the initial side effects wear off - they didn't. Didn't have any more and eventually had a non-hormonal coil (old fashioned copper type, not Mirena) fitted as hormonal methods all seemed to be really bad for me.
I absolutely detest condoms but we're using those now until it's ok for me to get pregnant again (had c-section so was told at least 3 months, preferably 6 to heal) and after that it'll be back to NTNP as it took a long time to conceive first time round so we're happy to go with the flow and see what happens.