I had the implanon a few years back, but had it removed after a year. I was one of the unlucky ones who had continuous bleeding. It wasn't for the full 12 months non-stop, more like on for 3 months, off for one kind of thing. The bleeding was very, very light though - kind of like on the last day(s) of your period. Apart from that, I loved it. No pill to worry about, and I had a health care card at the time so it cost me next to nothing for what could have been 3 years of contraception! The constant bleeding did become very annoying though.
I didn't have any problems with weight gain - in fact, I was trying to lose weight for my wedding at the time and I managed that fine.
I hope all this info is helpful to you. I think it's just one of those things you have to try and see if it suits you. Some women are really lucky and don't have their period at all.