The more variance there is within the sole of the shoe - by which I mean dual-density areas, footbridges, stability webs etc - the more the shoe will have the potential to cause aches and pains when it gets old. When it is brand new, everything is uniform in the sole, and as your foot lands it is supprted evenly, as it should be. As the sole ages, parts of it compress and parts of it - where there are tougher compnents such as the harder part of a dual-density midsole - don't compress so much. So where before you were landing on an even surface, after x hundred miles that surface is less even and your foot is subtly distorted each footstrike.

I track my shoes to the mile. I am a Brooks Adrenaline runner. I give a visual inspection often, listen to what the tootsies are telling me after different types of running, and at 300 miles start into a rotation of a new pair where I will wear my old pair for 1 or 2 "short" runs per week - after breaking in my new pair. I have some shoes that are gone after 350 miles and 1 pair that went 450+.

I see folks talk about Mizunos........what I have heard is they are VERY comfortable shoes but you will get less miles out of them than other brands. Sorry to make generalizations, but I have heard this from several runners and from sales people at specialty running stores.