Recent shootout showed Vegas and Liquid Chrome to give the best keys. As others have said getting a good key is the easy part, making it look like it isn't a key is the tricky part.
Vegas and Liquid Chrome? You're talking about a NLE shootout? Clearly there are better peroforming alternatives in the Compositing domain. The Cosby video was entirely composited in Combustion, clips exported from Avid XpressDV scene by scene.
