The underlying concept is good, but I am not aware of any gold backed cryptocurrency that makes it easy to actually exchange their coin for either actual gold or the equivalent value of fiat. That results in the coins generally not being anywhere near the value of the gold that supposedly backs it.  Consider for example that ZenGold is currently worth 1000 times less than the value of a gram of gold despite the fact that each coin is supposed to be backed by 1 gram of gold. 

DigixDAO has that same issue but managed to actually be worth significantly more than the value of the gold that backs it, generally 900%-1,200%.  This is partly the consequence of the fact that many investors don't actually understand what it means for a coin to be backed by gold. This fact has been repeatedly made evident in forums in which this topic has been discussed. The other factor in the success of DigixDAO is the listing on Binance. It has become a tradition to buy DGD when BTC/USD goes down, since gold is considered to be a safe haven. Demand exceeds supply during dips and buyers are willing to pay far in excess of the value of the gold that backs the coin because they don't understand what that means.