The engine rule seems to be exploited to the fullest already. Because BAR-Honda didn't have a chance to get any points in Australia, they drove to the pits on the last lap and therefore they have the right to change their engines for the next race. And Ferrari is considering about changing Schumacher's engine, because that way he could get the new car for the 4th GP of the season (and not the 5th as they originally said), because the engines between the F2004M and the F2005 aren't the same.
The tire rule is good because that really brings out the drivers who can really drive, but this engine stuff... It isn't worth it. The whole sport should get back to basics.