Body filler is meant to be used as a filler not a glue. More important then the product is the application. Even the best product will yield poor results with improper application. It is very important to get the right mix ratio and to mix the two part throughly. I use one speader to mix the two parts and another to apply the filler. But as mentioned by antimatter.....if what is below the filler is not solid the filler will not work properly.
As for products....anything from Rage is the bomb.