I went from a 23mm to 25mm tires on my '07 Tarmac Expert. The change was a blessing as I was able to do everything with more confidence. A part of it was for going from Gatorskins to Grand Prix 4000s II. Now that my Grand Prix are about to be worn out, I'm considering to going to 28mm versions. I know that my frame and brake callipers can clear the tire, however I'm not sure if I will gain any major benefits. I do a lot of climbin and try to stay off the flats.
I use 28mm in the winter when the roads around here in New England get bad. The roads are sometimes a combination of wet, sandy, and some patches of ice. The wider tires seem to help. I have more contact with the road I feel. The wider softer tires help with shock absorption with the holes that start to form in the road. I recall hearing during the TdF commentary a few years back that on raining days the riders lessen their tire pressure in order to increase the contact area between the road and tire. I figure that a 28mm does that for me all the time.
A couple of drawbacks of 28mm are the rolling resistance is a little more and I can't get the tire on the frame fully inflated. The 28mm are so wide that they will not fit through my brakes if the tires is fully inflated.
Someone asked above why one would need to inflate 28mm tires any less. I find this is the case for me. I attribute it to the tube containing more air so any air I do loose overnight is less noticeable. Essentially I get away with topping off my tires less often.