How can I effectively use the Coppersmith?
Coppersmith is a terminal action, therefore you have to design your deck to accomodate it.  (By terminal action, I mean it is an action that does not give +Actions).  As others have noted, Coppersmith is a card used to give you an early start.  By managing your deck, you can extend that advantage well into the mid game.

Here is a chart showing how 1 coppersmith can accelerate your early game.  I've compared it to Smithy, and "Big Money"  (A base strategy of buying only silver/gold/provinces which is actually fairly strong)


                    Chance of Having (once during the two turns)
Turn 1-2 purchases   $6      $7     $8       Notes
---------------------------------------------------------------------------------
Coppersmith+Treasure 50.91%   0.00%  21.21% # 1 in 792 that 2 golds can be bought
Smithy+Silver        42.60%  19.39%   2.12% #Not possible with a 5/2 copper split
2 Silver             33.59%   8.84%   0.00% #Not possible with a 5/2 copper split


This shows a few things


you have a vastly higher chance of affording a province.
The coppersmith gives you a 70% higher chance of being able to afford a gold than "Big Money"
The coppersmith works with a 5/2 copper split on the first 2 turns, while the other two do not.
Coppersmith will not give you possibly annoying $5 and $7 hands.


Is buying a province on turn 3 a good move?  Maybe not, but it could be.  My point is that if you buy coppersmith, you stand a higher chance of getting an early gold.  This advantage will carry over to future turns.  Coppersmith will help you hit the financial high points of $6 and $8 more often, while avoiding $5 and especially $7.

Every set of 10 cards in Dominion is different.  Coppersmith won't always be a great card, but it can be used to great effect.  Try playing it with wishing well!