I did test JANUS (it was called Project JANUS) some time ago and basically it just worked - mozilla, yum, other applications. I did try with CentOS.
From the announcement:
BrandZ is a technology that extends the zones infrastructure to allow for the creation of "non-native" zones. Non-native is a deliberately ambiguous term, as we are trying not to let our preconceived notions of the technology limit its usefulness.
The first brand we are developing under the BrandZ umbrella is 'lx', a brand that supports the execution of 32-bit x86 Linux applications on a x86/x64 machine running Solaris Nevada. Specifically, the lx brand allows the user to install a complete CentOS or Red Hat Enterprise Linux 3.x distribution in a zone. When the zone is booted it will still be running the Solaris kernel under the hood, but the userspace environment will include nothing but Linux software from init(1M) on up.
In theory the BrandZ infrastructure could also be used to create other types of zones. One such example would be a GNU Solaris brand, which runs Solaris binaries but has the standard utilities replaced by their GNU equivalents. Other possible uses would be the creation of zones for running FreeBSD or Darwin x86 environments. Because this technology is being made available via the OpenSolaris community, you as a community member will be able to help create these or other brands if so inspired.
If you are interested in a background information about BrandZ read it.