Hi Pamw,
I organized seminars for my department some time ago. I can tell you that it takes time, energy and hard work. This said, I think it's a great opportunity and, as far as I am concerned, I had the chance to network with people that I would't have the opportunity to get in touch with otherwise. I am really happy I did it. If I had listened to my supervisor I wouldn't have done anything. I think that you need to think carefully about the time issue, and if you think you want to go for it, do it!