There is ample evidence that 'knocking copy' is not only viable but can be a valuable way to gain new customers.

It is a very dangerous game to play in, though, if you don't know all the rules - the most important of which is being able to prove every comparison that you make.

One arena in which what you are suggesting is used is in the mass produced, moronic review site field where the guru selling the software to generate a comparison site suggests the you rate each of three products as good but rate the one you really would prefer to sell that little bit better. Not knocking copy but neither is it a true review.