You get one happier person, a little culture, and slowly expanding borders, for which you pay 60 shields (unless religious) + maintenance every turn. For 60 shields, I can build two settlers, each of which will temporarily reduce overcrowding in the cities where they were built. Each settler will expand the number of tiles under my control by a maximum of 9 tiles, but that's before I build any improvements. Then each settler will begin generating food, shields, gold, military units, more settlers, workers, unit support, beakers etc. I also have a great deal more flexibility with my gold/tech/luxury spending if I use the lux slider for happiness issues. Investments such as settlers (2 for the price of a temple), or military units (2 horses for the price of a temple) allow you to gain land, resources, luxuries (which help with the happiness). Most of my cities never get a temple. The exceptions tend to be coastal towns where I need to grab a whale or something, or, in very rare cases, border towns where the AI has lots of culture. (Truthfully, though, I usually just go burn some of their towns down and move their cultural borders).