I know roleplaying isn't about realism, and the coins are very pretty, but for some reason it bugs me that they come in denominations of the same metal. Doodz, there is no such thing as a "20gp" coin. Money wasn't backed by government guarantees, it was backed by the intrinsic value of the metal. To have a value of 20gp, the coin would have to contain 20 times as much gold as a gp, and have 20 times the volume, which would make it absurdly large. So 20gp was represented by, yes, 20 pieces of gold.
(Yes, you might be able to make, say, a normal-sized 20cp coin out of silver, if silver were 20 times as valuable as copper. But in that case it would just be a silver piece. And the values of metals fluctuate depending on supply, so if nearby miners struck a vein of silver, the value of your 20cp coin might go down to 15cp. Way too confusing.)
Face it, using "20gp coins" is about as silly as claiming your fighter can wear two sets of plate armor on top of each other...
But yeah, the coins are pretty, so don't make fun of me if I end up buying some anyway. :-)