I canít imagine there would be any beers from NZ brewed with Riwaka imported to the US, I think your best bet would be finding something brewed by a local micro using it.
I havenít brewed with Riwaka but Iíve brewed with plenty of other NZ hops, and they all have a fairly similar character, in the same way that all "C" hops have a similar character. To me they are very expressive and distinctive, usually tasting very strongly of tropical fruit.
If youíve tried any beers brewed with other NZ hops (Motueka, Nelson Sauvin, Pacific Hallertau) youíll probably have a rough idea how it goes.
Hereís the growersí info: http://www.nzhops.co.nz/varieties/riwaka.html
It used to be called "D Saaz" which should give an idea of parentage (even though it wonít taste anything like CZ Saaz).
If you want more info try doing a search for "D Saaz" or "Riwaka" on aussiehomebrewer.com. It looks like heaps of people there have brewed with it, but thereís no central thread specifically on Riwaka.