for 2009, i think it depends on the circuit. for some tracks with long straights, i think cars fitted with KERS will have an advantage. But on technical courses, it may be a handicap considering the added weight that also ruins the car's balance, etc. that's for 2009.
for 2010, it will be a different story. with teams developing the system and finding out new tech to boost its storage (currently it can store up to an additional 80hp), cars without KERS for the 2010 season will indeed be on a disadvantage. some say storage can be as small as an additional 130hp. maybe the "packaging" will be improved as well, lowering weight, addressing balance problems, etc.
IMO and AFAIK...
