I may have to install the beta and test before I sell my MacBook...
10.1 is definitely faster when playing h.264 movies. I used Vimeo as an example, and CPU use by Flash alone went from ~95% to ~85%. Hooray! It also seems like it might be more responsive playing other content, but I don't really do that because most other Flash content is junk.
Note, if anyone is thinking of trying it out: some websites don't work because they check that your Flash version is '10' and not '>= 10'. A friend who upgraded had to use a website that required flash, but was unable to. This is obviously a developer problem, not a flash problem, but be aware.
Oh, and you can't downgrade. Once you upgrade, you're stuck (unless you are willing to dig around).
In the end, I don't care. I already block flash because most uses of flash are obnoxious and irritating; now with 10.1, I can block a newer, more responsive version of Flash. Sorry Adobe, it's great that you're pretending you care and all, but it's too little too late. You've ignored us for years, you stuck us with a shitty Flash player and now you're telling us how lucky we are to have had it. I'm done with you.
[1]: http://kb2.adobe.com/cps/141/tn_14157.html