I had the LBS put 26"x1.25" to replace 26"x1.75" mountain bike tires so that I could tackle the hills in my neighborhood easier. It was a very noticeable improvement. But I also noticed that the trail I ride on the weekends, which is basically wood decking through wetlands, rode much rougher. No big deal, since I can head south on a paved trail instead. The other thing I noticed was that you have to be more careful riding over debris (like pine cones and leaves because the road tires will slide around on that kind of stuff easier.

GaPavedTrailRdr