I put 8000 or so miles on a SST. As others have said it is a stiff frame but I personally did not find it to be too harsh or hard to ride distances on it. The only gripes I had were original wheels were poor and the seat post clamp had to be near full torque and the post well coated in carbon paste or it would move down on bumps. The spokes on the rear would stretch and the wheel would go out of true. Performance/Fuji quickly replaced the wheel set with a better set. I later swapped in a set of Reynolds carbon clinchers. I found that wheels and tires made a notable change in the ride.

I swapped the SST frame for a 2014 Tarmac pro frame one evening. Everything but the frame, seatpost and fork went from the SST to the Tarmac. This let me compare frame to frame. The Tarmac feels a bit less "stiff" but not at all soft or in a bad way. They both feel like they are stiff and responsive when I crank hard and both turn in well.

Wheels and tires and a good fit go a long way to set the tone of a bike.