I have an 07 Synapse...aluminum w/105 components. It was my first road bike and I was fitted on it based on my desire to do just what you are planning on doing...general road riding for fitness and some longer centuries/charity rides. I'm taking it to the LSC Philly this summer. I'm not plannning on racing so it works well for me and you can still get plenty of speed. That's more about the engine than the bike would be my guess.

Will my next bike be another Synapse? Not sure on that because as I ride more my feelings may change but I still want a bike I can endure longish rides on. Just get what feels good so you'll keep riding it and not park it in the garage because it is "just not comfortable".