Flash's Actionscript opens up a vast field of possibilities. Programmers and designers have used Flash to create interactve features ranging from very lively feedback forms to attractive Flash-based games. This whole new level of interactivity will always leave visitors coming back for more.
A standardized site
With Flash, you do not have to worry about cross-browser compatibility. No more woes over how a certain css code displays differently in Internet Explorer, Firefox and Opera. When you position your site elements in Flash, they will always appear as they are as long as the user has Flash Player installed.
Better expression through animation
In Flash, one can make use of its animating features to convey a message in a much more efficient and effective way. Flash is a lightweight option for animation because it is vector based (and hence smaller file sizes) as opposed to real "movie files" that are raster based and hence much larger in size.
The bad and the ugly:
The Flash player
People have to download the Flash player in advance before they can view Flash movies, so by using Flash your visitor range will decrease considerably because not everyone will be willing to download the Flash player just to view your site. You'll also have to put in additional work in redirecting the user to the Flash download page if he or she doesn't have the player installed.
If your content was presented in Flash, most search engines wouldn't be able to index your content. Hence, you will not be able to rank well in search engines and there will be less traffic heading to your site.
Users have to wait longer than usual to load Flash content compared to regular text and images, and some visitors might just lose their patience and click the Back button. The longer your Flash takes to load, the more you risk losing visitors.
The best way to go is to use Flash only when you absolutely need the interactivity and motion that comes with it. Otherwise, use a mixture of Flash and HTML or use pure text if your site is purely to present simple textual and graphical information.