It's quite interesting what the drivers have to say on whether it's easier to pass or not, also how KERS affects the racing:
Rubens:
At one point, I thought I could overtake, it didn't matter if I had the wings to play with or anything, so it didn't seem to be easier but I think people did have problems, some more than others, with tyres temperatures...
Jenson:
I don't want to be pessimistic but I spent many laps behind Fisichella who was a second or a second and a half slower than me and I couldn't overtake, and I really needed to overtake him, so I was really pushing and attacking but I couldn't make it.
Those taken from the press conference after the race
http://www.autosport.com/news/report.php/id/74075
Massa in response to: What was it like racing with the KERS?
Massa:
I did a fantastic start but then it was quite difficult with the overtaking.
http://www.autosport.com/news/report.php/id/74074
Glock:
"It felt like I had a KERS (Kinetic Energy Recovery Systems) allergy! Every time I was behind a KERS car there was no chance for me to overtake. It was especially tricky being behind the Renault; I lost so much downforce, so when I see a Renault in front of us there is no change from last year in terms of more overtaking or whatever."
"However, when I was in clear air I was able to go about a second faster – so the performance is definitely there."
http://www.crash.net/Formula+One/News/1 ... again.html
Edit - Some additions from
http://uk.eurosport.yahoo.com/29032009/ ... dated.html :
Alonso: "With the KERS, I didn’t feel a big difference and I was a little bit disappointed at how it worked."
Heidfeld: "I think, at least, the KERS helped me to defend my position with such a slow car but, in the end, it didn’t matter much because who cares if you finish eleventh or 13th."
"It could be done manually. It would take quite a while, but it could be done. There is however a much more efficient and accurate way of getting the data. Men with lasers." Wing Commander Andy Green