They are not only NOT quick/effective flea killers but they can
make a cat sick.

So can Advantage.

Actually, in testing, Advantage was literally FED to the test subjects. With no ill
effects.

From the manufacturer website:
"Advantage is backed by extensive and thorough domestic animal safety studies that show
even when administered at five times the target therapeutic dosage for three consecutive
days, cats and dogs experienced no adverse effects4. If you apply as recommended, you can
be confident of the gentleness of Advantage on your pet.
4Bayer Domestic Animal Safety Studies"

The active ingredient, is also used as a grain pesticide, and is thereby ingested, even
by humans. You can read more on this he
http://ace.orst.edu/info/extoxnet/pips/imidaclo.htm
(snip excerpted from toxicity studies-note-the imidacloprid was ingested in these studies)
"Chronic Toxicity: A 2-year feeding study in rats fed up to 1,800 ppm resulted in a No
Observable Effect Level (NOEL) of 100 ppm (5.7 mg/kg body weight in males and 7.6 mg/kg in
females). Adverse effects included decreased body weight gain in females at 300 ppm, and
increased thyroid lesions in males at 300 ppm and females at 900 ppm. A 1-year feeding
study in dogs fed up to 2,500 ppm resulted in a NOEL of 1,250 ppm (41 mg/kg). Adverse
effects included increased cholesterol levels in the blood, and some stress to the liver
(measured by elevated liver cytochrome p-450 levels) (331)"

We use Advantage, and have had no problems, even on very sick animals.
D.