"Each card has a 2% chance of turning to gold immediately after being revealed."
This means that only about every 50th card you draw is a gold card. Now, if you're an unlucky guy like me who mostly draws potions that you're more likely to not be able to use as a beginner, then let me tell you this: be happy for every gold card you can get.
I keep all the gold cards I draw as long as I can rent them out well. I've only drawn one gold card so far, unfortunately. Twilight Basilisk. I really like the design of the card, but I'm not convinced of its function yet. What deck would you include the card in? Write it in the comments!
My favorite card is also a dragon card: Djinn Chwala. I would like to have this card in gold. I like it visually the best. The use against water with melee is really nice to look at, as well as against everything else that fights with melee attacks and becomes a victim of the attack itself through the thorns.
Twilight Basilisk I have pretty much permanently rented out because I don't know how to incorporate it into a deck.
Golden cards and heroes bring a bonus. Each card brings 10% when winning a game and the summoner brings also 10%. This means that you can play much more efficiently with a golden deck because the reward is higher when you win. Currently, however, it makes little sense to buy golden cards, so I rent mine. This way I get a small income stream and can hope for more good cards soon.