If you are interested in this debate I highly recommend reading Future Noir: The making of Blade Runner. It's a wonderfully detailed book about everything Blade Runner.

As far as Deckard being a Replicant, I always thought we was. There are many clues in the film:

- His collection of photographs

-At one point in the film his eyes glow

-After his final fight with Roy, there was a planned scene of Deckard in an Ambulance having his fingers set and his wounds attended to. On one of the displays behind him flashes "Replicant"

-How does Gaff know what Deckard is dreaming about?

There are others, but they are already mentioned in this node anyway.