What impact does the introduction of KERS have so far?

May be...

I'm not really sure. Its hard to say.
It was built up as a big thing for this years championship, but it seems that now you'd be lucky to find 3 teams a race that have KERS.
From what I have seen KERS has helped Mclaren when it comes to getting off the start line. But during the race it doesn't really seem to make any big difference.
Beginning of the year Red Bull had problems with the technology when Vettel was getting electric shocks from the system when he got out of the car. And I believe that in some cases the KERS system would interfere with other systems and cause it all to shut down.
I think that if they had properly developed the technology and not rushed it in then it might be in many more cars and be much more effective than it is at the moment.

That every team that concentrated of designing a KERS system, has suffered this season. Ferrari, McLaren, Renault and BMW's seasons have all been disappointing, where as Red Bull, Brawn and to some extent Williams and Toyota have benefited by concentrating on other areas.
It was a semi interesting idea that has fallen flat on its face. A lot of expense for a slightly quicker start and the ability to hold up non KERS cars, limiting the overtaking that it was supposedly meant to encourage.

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