Both environmental and money and other
The reason I switched to begin with was that it just hit me one time when I was paying $35 for a big box of diapers that was just going to go in the trash. I started feeling like I was "literally" throwing money away. I'm kinda glad I don't know how much we spent on sposies for the first 15 months. The environmental thing is big with me now, too. Alllllll those disposable diapers sitting there in landfills...at least when my CDs get to the point of being thrown away (after exhausting all other non-diapering uses for them), it'll just be a few dozen and not thosands.
And, the other: Cuteness! I use to hate seeing DS's sposies sticking out of the top of his pants, but I love seeing CD cuteness. Also, the health issues, more breathability, etc.