Pros and Cons to Flash
The good:
Interactivity
Flash’s Action script opens up a vast field of possibilities. Programmers and designers have used Flash to create interactive features ranging from very lively feedback forms to attractive Flash-based games and animation. This 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 Safari. When you position your site elements in Flash, they will always appear as they are as long as the user has Flash Player installed. However, with the never ending new versions, the user may have to download the most recent version in order to see the flash movie.
Better expression through animation
In Flash, you can utilize the animation to better convey your message. Nothing grabs attention like animated text or images.
The bad:
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.
Website optimization
This is in my opinion the biggest issue with flash, If your content is embedded in Flash, 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. Just as an image only shows up to the search engine spider, the same is for a flash swf file.
Loading time
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 SolutionThe best solution is to use Flash only when you need the interactivity and motion that comes with it. To stay search engine friendly and aesthetically pleasing, I have found that a mixture of flash and html or css is best for conversions and search engines.