Frankly, I'm not a big fan of buying precious metals in any form. I can justify maybe 5% of one's portfolio being invested in commodities generally, and only a portion of that being invested in precious metals. The pricing is too arbitrary to be much larger than that, but some presence probably provides additional stability to your portfolio. However, there is the right way to do it and the wrong way to do it.
As someone who collects coins, you might expect that I would endorse this as a way to accumulate gold and silver. Nothing could be further from the truth. It is precisely because I am familiar with this market that I can tell you that you should not do this.
Now, you will see infomercials selling a lot of this:
Case in point, is the 1909 gold Double Eagle with a Denver mint mark. If I've already lost you, you have proven my point already and you don't need to go any further. The value of this coin, with old gold values as of 2007 so you have to inflate these values, could vary between $1,850 if it was a MS-60 grade and $8,000 if it was a MS-63 grade. These grades can vary between coin rating agencies and there are a lot of phony agencies out there that provide inflated grades. If you buy at a premium grade that is incorrect, you can be getting yourself in a ton of trouble and you may already be out several grand without knowing it. Even reputable coin rating agencies like PCGS or NGC can be wrong, but you're safer if they've rated them. Generally, their word is... as good as gold. I hate myself for saying it, but there it is.
This brought up another point which is that someone selling you a collectible/historical coin may take advantage of your ignorance and not bother mentioning the mint mark. The mint mark is absolutely critical in the case of many coins as the total mintage at the various mints in this country and others varies widely as you might expect. Therefore, as a direct consequence, rarity is also highly variable. In the case of the coin mentioned above, a MS-63 1909 gold Double Eagle with a San Francisco mint mark is worth only around 12% of what the Denver mint mark is worth.
However, even assuming you are dealing with reputable dealers and not someone Glenn Beck pushed on you or some random E-Bay dealer that only has sold 6 items, you are essentially doubling down on alternative assets. You are subjecting yourself not only to the volatility of the price of gold and silver, but also to the trends in the collectibles market as a whole and specifically preferences regarding individual classes of coins. It may be that ten years from now, not only is gold down 40%, but that 1909 gold Double Eagles are out of favor in the collectibles market.
Coins that have little collectible value due to their commonality are actually better plays than the rare ones as there is less of a risk that you are bumbling into something that you don't understand and their is less of a squishy "collectible premium". However, the best bet is to buy gold/silver futures or just raw bullion if you want to make this play.
As for myself, I generally have stayed out of the high-ticket coins and have instead focused on coins that have interesting historical values to me. For example, getting the coin of a monarch in the last year of their reign before a revolution or some other event. You can actually get this reasonably cheaply if they are silver and they make for neat additions to any collection.