It installed fine on my Nexus One from bought from Google directly. Froyo is a lot smoother than Eclair. The most noticeable thing is that all transitions (opening apps, closing apps, opening up app list, sliding between home screen pages) have a higher frame rate.

Flash Beta is on the android store. Thankfully, the equivalent of FlashBlock is built into Android's browser - change the plugin settings to on demand, and you have to tap to enable Flash boxes. Flash ads etc. on pages generally murder scrolling performance.