As far as Bonanzle goes, still very early in its evolution. I've spoken to some over there that are happy with the sales, although they are mostly small time sellers.
Believe it or not, there are some people that just do not want to shop on ebay, and no one has ever lost business by adopting an effective market coverage strategy.
Ebay remains the biggest traffic site, but as the saying goes, the smallest fish in the sea are often the sweetest ones. Sites like Bonanzle, with their focus on community, along with providing an excellent selling platform will grow into the future, absolutely no doubt about it.
Will it ever be as big as ebay, who knows, but if it's a free option, then who really cares at the moment. If you can increase your market exposure for free, then what exactly is there to loose?
Come on people, it's a big world out there, and not everyone is looking on ebay, start expending your marketing of you want to maximize your sales!