I wore the Formosa Minnis houndstooth flannel suit a few times in the Bay Area this past winter when temps were in the low to mid-60's. It was great. Higher temps than that or too much sun even when the temp is cooler and I might have been uncomfortable. I'm not going to get a ton of wear out of it (unless the weather pattern returns to something more normal next winter), but I'm glad to have it.
Justin--you don't need black shoes if you have oxblood or navy, but you ought to have a pair just in case for all the reasons listed. Plus, if you wore them a few times with a navy or grey suit, you might change your mind.