One of my riding buddies has a Synapse Alloy 5 105 and he loves it. The Synapse Alloy 5 105 is the same frame but with a Shimano 105 drivetrain rather than the Tiagra or SRAM Apex that are standard on the Alloy 6. He got me to test ride an Alloy 6 Tiagra at his LBS a couple of months ago. It was a nice comfortable ride, and could have been fun if it had been my bike. You know how bike shops hate it when you return a test bike all scratched up/img/vbsmilies/smilies/rolleyes.gif. It just didn't inspire me the way my Raleigh or Centurion did the first time that I rode them.
Anyway, as always, fit is the most important aspect of a bike. Find a Synapse that fits and test ride it. It just might be the bike for you. Cannondale does make a fine quality bicycle.