All-flash sites are generally ineffective at achieving top placement (with a few exceptions). While search engines are getting better about reading the text inside flash files, flash websites themselves are not conducive to good usability for information rich sites. Search engine marketing is about delivering the most qualified traffic to the most qualified pages. When you optimize a website you want each visitor to land on the page that will provide them the information they were searching for. If you force every visitor to land on the same page (in this case a flash file) they will then have to navigate through the site to find what they want. That may be fine, but every click a user has to go through is an opportunity to lose that visitor before they find what they came looking for.
Flash Splash Pages
Many sites create splash pages with a flash animation which loads and plays before visitors are sent to the site. Not only do the visitors have to wait for the file to load, but they must also sit and watch the “movie”. There is no better way to cause someone to want to leave before finding anything useful than to force them to watch a move first. Adding a “skip” button lessens the aggravation only slightly.
Flash Embedded in HTML
Flash can be great for a website and there are proper ways to use it. The best way is to embed the flash into your HTML document. Make it a part of your website, not the complete website or complete home page. If you want your flash movie, then make is smaller and embed it on the home page so the visitor can watch it, or choose to read your content and click into other, more important areas of your site. If you think your movie has important information then you can always add a navigation link to it encouraging your visitors to watch.
Using flash as a part of your website can be a great enhancement. Just be sure that you’re not annoying visitors forcing them to watch (or hear) a flash movie they would rather not.