When I bought my fixie wheel set, they came with a cheapo set of 23s. After skidding and skidding and skidding (especially riding in Fresno with no brakes) the rear became very flat. Not flat like no air, but flat like not round. I figured I was getting close to the end of their use, and after 2 years of faithful, puncture free service, they were ready to re-tire. (oh, bad joke)
I figured since the roads were so nasty in Fresno, I'd try some 28s instead of the 23s. Why did I not do that sooner? Just some cheap Continental Ultra Sports, but man do they ever make a difference on bad urban roads. On all roads for that matter.
I've know this in theory, but this is the first bike I've ever actually change the tire width significantly on. I can't say I noticed any difference in my speed, but the road buzz, and harshness of cracks was definitely reduced. They still skid well too.