I've been on the injection for about 7 years now (don't think i've ever heard about only being able to be on it for 2 years at a time - no dr has ever mentioned it to me) I did take a break over the winter because i was working abroad so just saw it as a chance to give it a break. I've not had any side effect, it stops my periods almost completely. I occasionally get a little bit of bleeding but it will be exceptionally light and will only last for a day or two, but this only happens about once a year. Some people find that they bleed either as much as or more than normal in the first 3 months (ie before the second injection) this happened to me when i started it again in May but since i've had the next injection i've had no bleeding at all. I like it because i don't have to remember anything other than an appointment with the nurse every three months. With regards to the weight gain issue, no contraceptive makes you gain weight, some do however make you crave sweet foods which in turn could make you gain weight if you eat more than you used to. I get weighed and get my blood pressure taken everytime i get the jab and my weight has been stable the whole time, but then again i've never had much of a sweet tooth.
I've also had drs try to convince me to go on the implant, a year or so i moved house and swapped to a different surgery, the dr there nearly had me in tears and i almost walked out because she was trying to make me opt for the implant. There is some possible research to suggest that the injection can increase the risk or osteoporosis in later life. But that doesn't really bother me as i'm not in an 'at risk' group. I've since moved back to my old surgery and the nurse there is lovely, i used to have holiday injections from her when i was little and it hardly hurts when i get the injection.