Is some fish oil better than others?

I was listening to a doctor, Joseph Mercola, on the radio over the weekend. He was speaking about nutriotion and seemed to make some sense so I visited is web site.

On the web site he makes a case that some fish oil is much better than others and he just happens to sell the one which is best. I’d probably be a lot more likely to believe him if he just recommended it and told us where to buy it.

So, is this for real? Is the plain old fish oil I buy at the local drugstore the real thing or do I have to buy his special stuff in order to get the benefits?