I accept fully that many sites, including some I run, need advertising to operate. My Wi-Fi Networking News site has Flash ads on it right now, for instance. But I am, unfortunately, finding that Flash ads are now frequently highly intrusive even on sites I trust. They cycle endlessly. They use visual pows to knock my eyeballs on the floor. They play audio without my permission.
Since Flash is not under my control, I have to take a stronger action and use FlashBlock, a simple Firefox plug-in that loads Flash, but doesn't play it, putting a replacement symbol in its spot that, when hovered over, changes to a play button. This means that I'm back in control of my attention, sound, and browser.
If it catches on, it's another reason for IE users to switch to Firefox, and it will probably reduce the response rate on Flash ads, thus moving advertisers to other mechanisms.