Flash: 99% Bad
http://www.useit.com/alertbox/20001029.html

Flash is lousy because :

They delay users' ability to get what they came for. On the upside, most Flash intros feature a "skip intro" button. Why try to duplicate TV on the web and let the users skip it if they want ?

It encourages design abuse. Since we can make things move, why not make things move?

The Web lets users control their own destiny. Users go where they want, when they want. But many Flash designers abuse this and revert to presentation styles that resemble television rather than interactive media. Sites that force users to sit through sequences with nothing to do will be boring and pacifying, regardless of how cool they look.

Abuse of accepted standards. Many Flash designers introduce their own nonstandard GUI controls, forcing users to learn something new in every site they visit.

Usability issues

Flash being non standard HTML creates lots of problems :

The "Back" button does not work. So no undo for navigational mistakes

Link colors don't work. Creates navigational confusion.

The "Make text bigger/smaller" button does not work.

"Find in page" feature does not work. Flash does not support search.

Internationalization and localization is complicated.

Site's core values compromised

Flash screws up the site by

Not allowing frequent updation of content

Not allowing much content to be given

Not easy to create and deploy so gets created by outside agents who don't understand the business.