Depends on the design of the laptop. If it has decently enough vents on the bottom of the laptop, a laptop cooler can likely reduce temps by 2c - 7c. It also depends on where the fans are located on the cooling pad. If the fan is blowing air up to the laptop where there are no vents then there would be no affect on temps.
For uni-body aluminum laptops or any laptops with a metal underside, I think the cooler should help with reducing heat whether or not there is any vents directly above the fan since I would think the metal chassis itself should help dissipate heat.