This can be a gray area because the major search engines all have strict guidelines about using a competitor's name as one of your keywords. Their concern is legitimate, because users who looking for Brand X will probably be disappointed if they end up on a site for Brand Y. But the use of competitor's terms as keywords is permitted, and can be useful, if your website provides a comparison of your product or service to that of your competitor. Just remember, any claims of being "the best," or any other such superlative language, will have to be backed up factually, or Google and Yahoo! will reject the listing.