Back in the day (2000-2005), Oracle on Solaris was rock solid. There was basically no better platform or database configuration that was better than this, including SQL Server/Windows 2000/2003, etc.
It's a shame that Solaris is basically dead, but Linux killed them swiftly by being "good enough" on commodity hardware, and then Oracle did a coup de grace with their death-embrace by buying them. It's a shame, because so many great things were being done by Sun.
Oracle Solaris might be dying, and Linux sure has a large market share, but illumos as the open source continuation of OpenSolaris is doing very well.
If you enjoyed ZFS, Zones, DTrace and so on in the past there is a good chance that you'd like OmniOS, one of the illumos distributions for classic server environments.
There are more distributions like SmartOS that tkae a more innovative approach as cloud-hypervisor that is just a slim live system.
Most of the original ZFS Developers are now working on OpenZFS in illumos.
Beeing open source also means collaborations with other OS projects like FreeBSD (bootloader and more), NetBSD (pkgsrc) is happening.
The Oracle docs are not really the reference documentation for the open-source stuff, since illumos and Oracle Solaris have diverged quite a bit by now. Some things are implemented in different ways. OpenZFS in illumos has many features not in the closed oracle zfs.
The real reference docs are the man pages [1].
Depending on the illumos distribution there are more task-oriented guides for OmniOS [2], OpenIndiana [3] or the SmartOS Wiki [4].
In-depth information (driver development, MDB, DTrace) is written down in the "illumos books" [5].
These are not in danger from oracle.
Sadly a lot of good historical documentation is already gone since oracle broke all the sun.com links and not even archive.org has those. :(
It's a shame that Solaris is basically dead, but Linux killed them swiftly by being "good enough" on commodity hardware, and then Oracle did a coup de grace with their death-embrace by buying them. It's a shame, because so many great things were being done by Sun.