I'd just like to tack the following onto this discussion:
Some yeasts are extremely temperature sensitive, and you may have to control temperature quite precisely in order to get the desired results. (Hefeweizen yeasts, for example, can yield quite different flavor results if fermented a degree lower or higher. And many Belgian strains will get stuck if they go below 65 F)
However, temperature stability is generally much more important to good brewing results than absolute temperature control. Basements are great in this regard: even if it's a degree or three above or below the ideal temp, steady temps over the course of active fermentation will yield maximal results.