Hmm. Don't really see many "bad reactions" to Frontline to be honest. I think most of the descriptions above are due to the irritation/itching from putting something oily on their skin (try washing a cat with anything and I reckon you'd get a bad reaction 🙂 )
The thing to do is maybe try with an alternative (from your vet, NOT the supermarket – Frontline is the ONLY effective flea treatment you can get from outside a vets) and see if they respond better to that.
Clubber – flea combs are like weeing on a forest fire – you'll kill the odd adult (the frontline should do that anyway) but do nothing for the underlying cause. Assuming you've used frontline (or similar) monthly and something like acclaim or Staykill in the house, I reckon a trip to the vet may be in order – you sure it's fleas?