Seems to be very hit and miss for the flash player. Pretty much just shows adverts on webpages in my experience.
It may require a Flashplayer version higher than that in the Hero. Adobe is set to announce a proper mobile version of Flashplayer in October that is more current. I believe HTC worked with Adobe to get one that worked with most sites - at least Adobe were very proud in demonstrating the HTC Hero capabilities.
The flash implementation in phones isn't full Flash 10, it's a cut-down version, so some things might not interact well with it. Some websites work, some don't.
