Grillo means cricket - the insect - in Italian, but can also mean caprice: avere grilli per la testa means to have a head full of crazy ideas. It is bizarrely appropriate. Grillo says correctly that the party leadership is useless and that it is corrupt (also correct.) But he is not suggesting new policies, he is against everything.
A good example is the story going round that the elections have been put back a week so that new MPs if voted out can get their pensions (they need to serve exactly 5 years). Whether true or not, everyone believes this of the political caste.
Grillo is doing well in the polls because that is how the Italians are feeling: they are fed up with the venality of public life, which they could tolerate when times were good but now they see they are paying for it.
It does not seem likely to me that this discontent is going to change under the present lot, left or right. Italy has six months to find a credible, strong leader who can explain to the people what is going on and help them through it. It isn't Grillo: he is the catalyst.
The markets are watching.