The one good thing about KERS is that it could/should help overtaking, i.e. giving a boost to overtake at critical moments (though of course the other driver could in theory respond and use his own KERS to defend, but that'll depend if it's charged up, if he reacts quickly enough etc).
For anyone who missed the quali, apparently both Ferraris, both Mclarens, both Renaults and Heidfeld's BMW are the ones running KERS this weekend, no-one else is. At one point there was a graphic to accompany the on-board footage which, as well as showing the traditional revs/brake/throttle graphs, also had a KERS bar which showed how much charge there was in it, and you could also see (Raikkonen I think ?) using it at several points in the quali.
Fantastic news that Brawn do seem to have real pace, after all their problems. Though of course it remains to be seen how much fuel they're carrying into the race.