I think more people talk about CAAD because it's a catchy trademark and Cannondale's aluminum race frames have proven to be really good bang-for-the-buck. In the past, the CAAD series were indeed pro-level race bikes. You just won't easily find a better aluminum frame in terms of efficiency and handling.

The Synapse, IMO, is still plenty of bike for almost everyone reading Bikeforums. Picture it alongside other manufacturers' Roubaix-type bikes, intended for making rougher pavement less annoying (Cannondale's sponsored teams use, or used, it for the Paris-Roubaix race, for example). It's not gonna have rocketship-like acceleration, but it'll be a little nicer on longer distances.

I had a previous-generation CAAD8 and chose it over an alloy Synapse because I wanted a faster, harder-riding road bike so I could pretend I was good. Looking back now, I think I would've been just as happy with the Synapse.