Creating a all Flash site has its share of cons. Some things are harder to update and maintain. People without Flash have no way of accessing the content. It takes longer to load. It's harder for search engines to index. You can't bookmark certain sections of the site. (since everything is pretty much happening on one HTML page)

For a client, I would never create an all Flash site for the above reasons, but this is just my personal site for entertainment. I think the biggest pro is aesthetics. My site will look the same no matter if you are using IE or Mozilla, Mac or PC. Also, a lot of the button rollovers and page transistions could never be done with done with an HTML site or an HTML/Flash hybrid.