I ordered a medium size Furminator for our dogs and cats about 4 months ago and I LOVE it! It works wonders on our cats, I can see and feel the difference afterwards.
It's fantastic on Zoey my Alaskan Malamute. Huskies, Mals, and other breeds with water-proof guard hairs and short undercoat like nordic breeds are PERFECT for this brush in my opinion. When Zoey was blowing her coat, the Furminator helped immensely.
I too have heard that on really long-coated breeds, this brush doesn't work too well. On Dolly my Saint, it works okay. Just okay. It helps a lot on her belly and the fur near her back legs, but I usually prefer our slicker brush for her.
Furminator would probably be okay on Rotties and I have a friend who used it on their Lab and the dog loved it and the fur was flying! But dogs with really short fur like Danes, GSP's etc. probably wouldn't appreciate this brush.