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I really like Bazaar Voice for their on-site review integrations. They spoke at GR2L during SXSW last year and I was very impressed with their offering.

If those pages are indexed by Google and Google returns them in SERPs then yes, they will 404. That is why you need to test the page first and do a header redirect 301 to either the category page or the home page. Hope that was the This Answered My Question : )