So, it appears from some early reports on the S5 and Foil that they are stiff and fast, but perhaps not very comfortable or compliant at the rear.
Would an S3 be more comfortable?
Are these bikes a niche play for marketing and sales, or is there a real place for them? Seems to me that the majority would still be best suited with an all around bike rather than switching or trying an aero bike?
A Tarmac SL3 or SL4, Cervelo R3 or R5 seem like they still offer an overall better package for all characteristics?
Yes…I think that everyone I race bikes against should think EXACTLY like this 
Honestly, aero isnt everything as much as ST tends to disagree. Despite aero frames being available, many racers are still remaining (and winning) on standard road bike designs. Aero may help, but 99.9% of the cycling game is the athlete, not the bike.