Conventions that are scams are pretty rare. If there are a bunch of famous writers that you want to meet, chances are it's not a scam. Off the top of my head: Maui writers con (http://www.mauiwriters.com/) and Pikes Peak (http://www.ppwc.net/), Surrey (http://www.siwc.ca/), and Willamette (http://www.willamettewriters.com/).