Okay, I'm going to get the ball rolling, since no-one has posted their tips/hints/explanations yet.

Bear in mind: I'm about two to five days away from getting the game (if I don't break down and hit Walmart tomorrow night... only to find they're sold out...).
Speculation: Multiple religions can be beneficial.
If you found more than one religion, each gets a holy city (with possible overlaps?). Each religion can then build a shrine. Shrines get you money and culture (and possibly great prophets?). If you spread each of the religions to all your cities and to all foreign cities, the benefits of the religious shrines applies to each city for each religion. (Founded three religions, and have three shrines? Get those three religions in ten cities, and you're reaping the benefits as if you had one religion in thirty cities.)
I do not believe that there is unhappiness from having multiple religions in one town, only if the state religion is not in the town (?). And if you get freedom of religion, that's good for you, too.
What are the costs? Well, it takes a great prophet to build a shrine, which means you've got to put in the work to coax a great prophet to be born for each religion. (Each great person increases the cost of the next great person.) That means a lot of religious buildings in each of your holy cities. You've also got to build missionaries of each religion, and spam all your cities multiple times over. You've also probably had to focus research on religious techs, at the expense of absolutely required other techs (e.g., techs to build farms, roads, mines, and so on).
Also, you can only have one state religion, so even if you spread two or three religions to neighboring countries, they might choose the wrong religion. And switching back and forth between religions could have diplomatic consequences, since I believe the computer remembers your past actions.