Even if it's not included in Android, the Flash runtime is one of the few applications that require Android 2.2. Right now, you can install from the Android Market the first beta release for Flash 10.1.
Whether you love it or hate it, installing Flash changes the way you look at a mobile phone. HTML5 may be the future, but a lot of websites use Flash for playing video, music, games and interactive content. Instead of getting messages that recommend you to install the Flash plug-in, you'll see the actual content.
The trouble with Flash on a mobile phone is that most Flash content is designed for a computer and it's difficult to use on a device with a small screen. Video players have small buttons and it's challenging to click on one of them, some websites serve high-quality videos that aren't appropriate for a slow Internet connection, clicking on a Flash object is a disrupting experience because you might open a new page, pause a video or display the Flash content in full-screen.
I've tried to open many sites that use Flash and the experience isn't smooth. Animations are sometimes choppy, web pages load much slower, scrolling web pages that use Flash is slow and there's a lot of lag when zooming a page with Flash content. In some cases, the browser is no longer responsive for a few seconds and you need to wait until you can switch to another page. Fortunately, Adobe managed to optimize the code and using Flash doesn't drain your phone's battery much faster.
The version you can install from the Android Market is not the final release, but don't expect too many changes until next month. It's nice to have options, so I recommend to install the Flash runtime and to change the browser settings so that plug-ins are loaded "on-demand". This way, web pages will continue to load fast and you'll only display Flash content when necessary.