B737-(NG) Winglet Performance/Fuel Burn

[MFALK]

Apparently, Boeing promises a 6% less fuel burn benefit with the new winglets installed on their -NG 737s.

Rumour through the grapevine has it that the gain is more in the region of 3% or even less, as experienced by the new customers.

Can anyone shed any light on this argument, preferably based on actual expereince?

Thanks.

MFALK