They may look flashy and "cool," but flash-based websites don't cut it online. Good and honest web designers know this and will warn you ahead of time. Behind all the special effects and jumping content, structurally they are flimsy and "cheap,” without being literally cheap ($).
Aside from the fact that you're shutting out all the people that didn't even install flash on their computers, you're alienating people with disabilities (yes, even blind people read web pages), and you're telling Google and other search engines to buzz off. So before falling in love with the whole idea of flash, take a look at some of the disadvantages and see if they conflict with your goals.
Flash sites are SEO-challenged when it comes to search engine visibility. If you want your site to be analyzed and indexed by search engines, your site needs to be composed of actual text, not graphics with text. Search engines do not read images the way people do; they scan text and read image file names and “alt” tags (image tags) at most.
Furthermore, visually impaired people read websites in a similar way. The text of the website is read to them out loud by special software. Therefore it is important to ensure that web design accommodates their needs and is universally accessible.
What are Flash sites good for?
Flash websites are good for certain niche markets that people don’t really search for via keywords, like a novelty products or services. Flash is also good for private trades that already have established clientele and don’t care about further marketing.
But I want my site to look cool..
You can still jazz up your site with flash, just don’t use flash as its structural foundation. Instead, add individual flash elements to an html/php-based website. That way you'll get the visibility of html combined with the aesthetics flash.