The material used by genuine Crocs seems to last much longer. My brother bought some knock-offs at the same time that I picked up a new pair. He wore through the soles in about two years while mine are still kicking after almost 8 years of near-daily use.

You can get the genuine ones for $18 to $35 on ebay depending on the color, so while I'm sure you can save some on clones it's not worth it for the durability and comfort.