Hi,
I rode the madone and domane back to back before I bought the domane.
The isospeed makes a huge difference (or did to me), the same roads felt like the difference between riding a race bike and riding a sofa, but didnt have any appreciable effect on speed.
Ok, not that different, but thats how the bike mags would put it, but yes made a big difference, hence the choice over the madone, I also have a back issue following an operatio, so having the correct size means it may look huge, but the overall effect of it all was well worth the cost!
also did my first test ride up to Potters Bar which has some crap roads and steep hills and it seemed to work really well - but I need to get fitter to get the full value