I have only ever been 'grilled' once, it was off a medium pace bowler and i was keeping and standing up to the stumps...
as a very inexperienced keeper who is usually a quicky, i took my eyes of a ball that was edged off a cow corner slog and it hit me flush on the grill.
From what i experienced the ball defo accelerated off the bat and came at me alot quicker than i expected...
I have the grille on the setting that shouldn't allow the ball through and it didn't come through, i use the Masuri that Kieswetter was wearing and it hasnt really put me off. However the fact a helmet that apparently is up to the new standards has failed has made me want to upgrade to the new Masuri...