Of course that 4S is worth a solid $350 on the used market, so the choice is yours.
That said, I think the majority of people generally stick to every other release for an upgrade.
I use two phones, so I'd probably make the 4S as my secondary phone and actually sell my iPhone 4. I've really had to talk myself out of the iPhone 5. It looks like a great phone, but I don't need it for any other reason than I want it. I'm happy to wait for the next model.