What has everyone's experience been having multiple domains pointing to the same site--good thing or bad thing. For example, if you have a website that sells used cards and the original site is crazysam.com, is there an advantage to also purchasing and pointing related domains to the same site--used-cars.net, etc. Just wondering.
What you're referring to is called "vanity domains", and it's not a useful SEO strategy. The search engines will see your sites as duplicate content, and typically will only rank your main domain. A exception to this is if you create regional domains (.ca, .co.uk, .nl, etc) as the engines should rank the appropriate tld for each country's localized search, especially if you use google webmaster tools to indicate each site's country. Pick one domain and stick with it. Create different pages for each term you want to focus on, and it will work better than vanity domains. For example, create a page at http://crazysam.com/used-cars with content about used cars.