They were sentient beings. What leads you to think they feel nothing? Their later generation models that we see are certainly feeling beings, it's one of the ironic points that they are more emotional at times than the colonials.

The old model Cylons we see in Razor keep a prototype hybrid around, and worship him as a god, and he does have a prophetic channel to whatever the Cylon god is. So why do you think they're unfeeling beings or have no sympathy for their slavery?