If you've been on the Depo for over 3 years then any problems that you may have had so far will probably remain constant. In other words, if you HAVEN'T, had problems with it, then you probably won't experience any in the future. However, if you miss your shot, then the symptoms can be just as erratic, as your body tries to re-balance your hormones to a "normal" level, as the symptoms can be when first starting the shot. I have also learned that, while Depo DOES increase your chances of developing breast cancer during the first 5 years of use, after that period, you become LESS likely than average to develop it.
I am not in anyway trying to persuade you of your decicion, but I feel that you should know the facts. By all means, don't blindly accept hearsay, but go out and search and do some research yourself before you make a major decision about your body.