More information on the 'new' exploit here;
http://isc.sans.org/ May not be too serious if Mozilla's view is correct?
As an Opera user I can afford to take a relaxed view about this and no, I'm not suggesting that Opera is better or safer than other browsers or that it can't be successfully attacked.
As for how many sites use JavaScript, it's going to be a lot, certainly many more than those that require Sun Java!