The injectable contraceptive is called Depo Provera. It is a progestogen-only contraceptive which lasts for 3 months at a time. Its really good if you want reliable contraception that you don't have to remember to take every day and is good for someone who has suffered from heavy or painful periods, as most people who use this form of contraception end up having no periods at all (usually after about the 2nd injection).
The problems associated with the method are the same as for any progestogen-based method (the 'mini-pill', Mirena, Implanon) and those side-effects include change in moods, acne, and erratic bleeding (especially in the first 6-12 weeks of use). The main problem though is unlike most other forms of contraception it cannot be withdrawn once given, so if you're one of the unlucky ones who suffers from a drastic change in mood, or erratic bleeding, etc, you just have to wait the 12 weeks for the method to wear off.
This is a method of contraception that women seem to love or hate! The important thing to remember is that it can take a while for your fertility to return once you have started taking this mthod - even if you only have 1 injection, it can take anything up to a year to get regular periods back again, so this is not a method to use if you were planning to conceive in the next 6 months.
Despite all this, I would say its very popular, but it is a method that you have to just try to find out if it suits you or not. You can't really go on friends experiences, because some of them will get on very well with it and others will hate it. HTH