Same experience here with my X300 notebook card. FPS in glxgears around 250 without beryl and jumps up to 4500fps (!?!) with beryl/xgl on. (it even looks like it laggs and I just can't imagine that this number is all that high, might be a bug?!)
I also receive extremely high cpu-loadage, it goes to 99-100% when watching a fullscreen video without doing anything else. (my notebook fan didn't even stop moving
I also got the info that calculatings should be done from the graphics now instead of the cpu, anyone have any infos on that or if it is different from xgl to aiglx?
btw: right now i run a fresh installed bianca and do not intend to isntall any stuff like beryl untill it is more stable and known to work mithout major glitches... well... at least I will try not to read a nice guide how to install all those nasty effects...