Update 2015-02-05: Since Safari 5.x hasn't been supported in years, I switched to using Chrome and Firefox as my primary browsers several years ago. However, aside from that detail, the rest of this still applies.
I don't like Adobe Flash. I don't have anything against the concept, and it does offer some neat, and occasionally useful capabilities that are difficult to implement using other technologies. However, I got really tired of the need for constant security patches for Flash, the Flash based ads, the performance slowdowns, and the reduced battery life due to all the Flash based content on websites and in ads.
So for the past 6 months, I've gone Flash free on my Mac. To be clear, I'm not really Flash free, I do have Google Chrome with it's built-in (and automatically updated) Flash, but as you'll see below, that's my backup option for those occasions where I really want/need to access some content that requires Flash.
I mostly use Safari 5.x and find it very quick, that's the main reason I switched from Firefox a couple years ago. I deliberately don't have Flash loaded on my machine, and that helps with performance and battery life. When I encounter a site that requires Flash, I either use Chrome (with it's built-in Flash) or I don't use that site. This is simple when you enable the "Develop" menu in Safari, one of the options on that menu is to open a page in any of the other browsers installed in your machine. Not everyone will like that setup, but I've been using it for about 6 months and I like it. Here are instructions for setting it up.
I use two other Safari extensions. YouTube5, which tells YouTube to send HTML5/H.264 video rather than Flash video, and ClickToPlugin, which makes all other plugins require you to click on the content before it loads. That keeps plugins from slowing down my machine and using extra bandwidth. Here are links to those two.
Also, since websites logs will show that I don't have Flash installed, it may eventually make them rethink their use of Flash, especially if a lot of people start doing this.
What I end up with is clean, fast browsing, no Flash ads, no extra junk slowing down my machine or using up battery. The trade-off is the minor inconvenience of having to occasionally click on some content to display it or open a page in Chrome. It's a trade-off I'm willing to make. Try it out for a couple weeks, you might find you like it.
2011-07-11 - Flash Player "zero day" vulnerabilities exploited in the last 30 days = 3, this year, 6-10. Flash Player is insecure, inefficient, and a high profile target for people trying to compromise your computer.
2011-12-14 - Adobe Flash Player vulnerabilities by year.
2011-12-30 - Mac users, use Flush to remove existing "persistent Flash cookies". Alternatively, you can manually delete all your Flash preferences by deleting this folder:
2012-01-05 - Safari 5.1.x has significantly more issues than 5.0.x, so after a couple weeks of using 5.1.2, I reverted to 5.0.5.
2012-01-29 - My Windows machines have been using a similar setup for some time. However, Safari for Windows is distinctly inferior to Chrome and Firefox for Windows, so I have standardized on using Chrome for my primary browser for Windows. The Adobe Flash built-in to Chrome 16 on my Windows XP machine was crashing multiple times per week, so I disabled the Flash plug-in and am operating completely without Flash on this machine for now. Since I will need Flash for a few sites, I will probably download Portable Chrome and leave Flash enabled there. That's not as convenient because Chrome doesn't have a handy menu option allowing you to open the page in another browser, but copy & paste of the URL works well.