If you're selling a product, is it best to tell a story of why it's useful or just to show what it does and let the product do the talking? Personally, if I go

If you're selling a product, is it best to tell a story of why it's useful or just to show what it does and let the product do the talking? Personally, if I go to a site and I have to sign in/up to get to the application, then I'm less likely to do it than if I was immediately logged into a guest account, perhaps with some pointer bubbles explaining what to do/what you can do.