I rode a Domane 5.9 with Ultegra Di2 for about 260 miles in CA a few weeks ago. Surfaces varied from smooth roads, to beat up pavement (Drum Canyon) to rock and gravel (Figueroa) and terrain included significant climbing (Figueroa, Jalama beach road). Distances ranged from 20 to 75 miles. My favorite bike is my full carbon Torelli Montefalco and I own a titanium gravel/winter bike, and a Kestrel Talon and I've owned a bunch of steel frames over the years. I've also rented Madone 5.2, carbon Giant TCR, two SuperSixes, and a Scott CR-1. I'd rank the Domane somewhere in the middle of that group. I think the combination of the more upright position and the low BB just made it feel sluggish and unresponsive. The IsoSpeed coupler didn't seem to offer any significant improvement over my Torelli which surprised me, especially since the Domane was running 25mm tires and I normally run 23mm Veloflexes. If I were looking for an "Endurance" bike, I'd pick the Scott CR-1 before the Domane.