I bought one for my two longer haired dogs and it works pretty great. Only downside for them is that you gotta be careful around the belly because the "bristles" are very hard, semi sharp, metal. So the angle you use it at makes a difference here.
I used the same Furminator comb on my cats and omg it does a great job of stripping the undercoat. It really improved the look of the cats after and the amount of shedding they do now, especially in the summer months.
Would definitely recommend a Furminator.