Expired domains may not be always the best option to go for. It depends on a lot of factors and of course most importantly your usage of the same. 1. A good domain name that can be marketed and also includes the niche keyword 2. A good previous PR and online reputation 3. Good rankings and traffic ... and many other factors that might help You have to do a lot of research on this. It might help you a lot in your campaign and might not help at all or even sometimes hamper your new campaign because something was unethical previously. You just have to be careful.
Google likes sites that are older so an expired domain name has a better chance of getting ranked well and quickly.
Advantage is they have accumulated years of backlinks. For example if facebook.com expired and you bought it, you would get the benefits of 5 years of multimillion dollar marketing.
i too agree with janessecret!! I too think that a only advantage of expired domain could be its age as google likes older sites.