I don't know enough about HOF to say whether it is considered a "designer" brand. In my country, it is not really as big as Tiffany, Cartier or Bvlgari as it does not have any stand-alone stores, but some jewellers do carry their range of products. HOF diamonds are considered premium diamonds where i live due to their superior cut, etc, etc (they are alot more sparkly than the regular diamonds), so they are much more expensive. Most people here who buy HOF diamonds go for the engagement rings or solitaires.
Thanks for sharing your knowledge!