With fglrx (HD 6550, i5 480m) flash 11 beta on 64 bit is totally unusable.
I had tried both in /etc/adobe/mms.cfg: With and without
With flash 11 no difference whatsoever: CPU between 80-100% in normal mode and when putting to fullscreen the screen freezes often for several seconds (!!) and exiting fullscreen takes about 10 seconds.
OverrideGPUValidation = 1
Now I use 10.3.181.34 with nspluginwrapper and it is much better. Without the entries in mms.cfg it is really bad, but fullscreen with them is actually really good in terms of cpu usage. Not as good as with a native player, of course, but that's the "best" videoplaying with flash I have ever seen.
That said, that often doesn't apply to sites other than youtube. On videohosters like videobb even a paused flash video will produce ~80% cpu usage.