What's so great about meditative experiences?

It's commonly held that meditative experiences are great for your brain and mental well being.

To me this elicits a number of questions?

1. what happens to the brain in meditative state?
2. what counts as a meditative state?
3. how does this link to personal faiths and religions?
4. how does this effect happiness?

I'm starting a series of entries that investigate these questions.