Seth Godin think search engines still have a lot to room to grow when it comes to comparison type searches:
Seth's Blog: The haystack
But they're terrible at connections, at rankings, at horizontal results. They can't help me find the 25 most important up and coming artists in the United States. They can't help me find six products that are viable alternatives to something that was just discontinued. They can't help me rank the service of four accounting firms.
He raises a good point. I get the impression that he expects Google to understand that a product-specific search for a product that's discontinued should generate links to alternative products.
This is something Amazon does today. I'm sure comparison shopping sites handle it as well.
One area where it's poorly done today is real estate, where most real estate sites today remove listings once sold rather than pointing people to related properties that are still on the market.
Using search engines, a person could run queries that generate relevant results as well. Try coming up with comparative phrases that may be mentioned on product review sites or blogs to generate appropriate search results. Here are a few suggestions to get you started:
Terms like that - especially when used in quotes - do a great job filtering search results down to sites with opinions on products. Give it a try.