I can only speak to the IsoSpeed (front and rear) on the Trek Domane - no, not a gimmick. If you're looking for comfort and stability, it's the ticket.
I usually run 28mm or 30mm tires and they do a nice job on muting some of the road texture, but the IsoSpeed does a fantastic job of absorbing the cracks and seams and taking the edge off of those big hits that you didn't see coming. Many that haven't ridden an IsoSpeed Trek will say that bigger tires will offer more compliance than a frame ever could, etc, etc. That's incorrect, in my experience. My gravel bike, with 38/40mm tires, and running at half the tire pressure, doesn't take big jolts as gracefully as the Domane with 28s. With good tires, the Domane is a magic carpet ride that's not bouncy or obtrusive.
That said, I recently bought a Cervelo R3 Disc - it is excellent. It's lighter and more responsive than the Domane (same wheels/tires) and provides great feedback without being harsh. I haven't had an opportunity to take it out for a 100+ mile ride, but I've gotten in a few in the 50-65 mile region. No, it's not as comfy, but it's comfy enough and the more nimble ride is a lot of fun.
I don't think that I'll keep both the Cervelo and the Domane - there is some overlap between the two of them and though the Domane would be more comfy for the 8-10 times per year that I do 80+ mile rides, the Cervelo isn't exactly a slouch, either. If I did more long rides, or longer long rides, I probably wouldn't think twice about keeping the Domane around, but as it is, I'll probably let it go. On the flip side, if I didn't do as many spirited group rides, I'd be happy with just the Domane.