Just as good as bottom baits Tony just another method of presentation.
Especially good in waters with silt or weed on the bottom.
Some people say that if fished with free offerings the fish may avoid it as it is different to the bottom baits surrounding it but loads of big carp have been caught on pop ups like pineapple or tiger nut flavour.
I like using the 'chod / helicopter' rig with pop ups, it is a well camouflaged rig and tangle free. Using a brightly coloured boilie or a strong smelling low oil based one will be good in these cold conditions especially at night.