I've had the Implanon implant since late August 2008 and although I wasn't on SW at the time, I didn't gain more than 4 lbs (at the absolute most) between then and January 2009 when I started SW. Since then, I've lost about a stone, but I eat way more than 110 syns a week and am a slow loser anyway!
It's not for everyone though as some people's bodies just don't get on with it (not just talking about weight here) but I've been fine. Give it a go and if it doesn't work well for you, the active ingredient is out of your system almost as soon as the implant has been removed.