I've done WW at work on and off for years. What I found was a downfall was that people get bored with it after two or three weeks and never come back. Those that are into get left in the dust when the session ends and they are on their own and need to find an offsite meeting because they no longer have the minimum amout to lose. That all said, it's great to have someone come in and it's right in the office. Like everything else in life those that want to do something about their health will!
One other note, check with you HR Benefits department, depending on your health care plan, weight watchers might be a refundable expense through your health care plan (at least a portion of it).