I haven't tried any of their other products, but the iMX6 has a very large detailed technical manual (publicly available, not under NDA) and their Linux kernel trees are updated in public via git (presumably not everything is public but it's a lot more transparent than the occasional tarball source drop approach), plus they have people involved in yocto development. Better than many/most of the ARM SoC manufacturers.
The only undocumented/secret squirrel business AFAIK is the Vivante GPU. And that's universal across ARM Soc GPUs, sadly.
I've built the Linux kernel and Android AOSP/ICS directly from their git repos and it all runs just fine. You can ask support questions on http://imxcommunity.org but the response you get will be sporadic. Your best bet at getting answers is to be a real customer and get an FAE to help you out. But otherwise I've never had a problem with a lack of "openness" from Freescale.
The only undocumented/secret squirrel business AFAIK is the Vivante GPU. And that's universal across ARM Soc GPUs, sadly.