Gold coins can be a good investment, because they are durable (can withstand fire, water, or trauma), are pleasing to caress, and can be used to fund getting out of town in a hurry. But all forms of gold have a markup, so probably the best form for investment is semi-numismatic gold coins. The biggest disadvantage is that it takes a little expertise to buy them efficiently.
What about bars? Are they better or worse than coins? I think the markup should be less, so probably more convenient. Or am I wrong?
I use to work here, call and ask for Dave (he's the Director of Metals operations). When I was there we had some of the lowest prices around. I'm sure most everyone knows this but in physical metals volume helps dictate price. http://www.alaronmetals.com/
The biggest problem with gold bars has already been stated -- they could be gold plated. You can "help" protect yourself by buying from a reputable dealer, but you might have trouble selling them, especially in an emergency (i.e. the apocalypse). To sell them, you may have to have them assayed, which takes time and costs money. Coins are generally more recognizable and harder to fake. The other consideration is that the last time the U.S. made gold ownership illegal, there was an exemption for collectable coins.