Many SEO experts believe buying an expired domain is useful as it may save the other expenses. An expired domain will be registered with Dmoz or several other directories and search engines and it will be easy to have a Google ranking, as the domain’s age will be more.
It is believed that expired domains bring with them the previous site's page rank and traffic. But it is not true as soon as a site expires all existing links are no longer counted towards the site resulting in its low page rank. You will have to work on it afresh just like working on a new domain. It may even happen that you may eve fail in your attempts to re-establish PR after purchasing the expired domain and registering.
Instead a domain can be purchased before it expires so the backlinks and PR are intact. Of course, it will be costly but a proper deal can be easily cracked. Definitely more traffic can be attracted depending on the popularity of the name in terms of it's relation to search traffic than if the site was new.
Two thinks should be considered while buying an expired domain:
1. The purpose of the domain name whether you want to purchase because it relates to your site or because the previous site had high traffic. Now if the site relates to your site it is fine but when it does not you will not be benefited from the traffic or off site SEO work.
2. Take care to see if any black hat method has been used to increase site popularity as it will definitely show its’ ill effects later on.