They r good, I have one and find it very effective. I do however still use my trusty wide tooth comb and slicker just as much though.
Also furminator is no good on non shedding dogs so spaniels, poodles, schnauzers,westies etc wont find it of any use.
They are quite expensive also. Dont no if any good but lazer blade rake by laube looks very similer and half the price.