Images can be downloaded from freenas downloads section. The best improvement, I think, is that the jail plugin for any plugins you want to download is now included in the install image and does not need to be downloaded and installed separately.
FreeNAS 9.1.0-BETA has many major improvements:
- Version 9.1.0-STABLE of FreeBSD with performance improvements, bug fixes, and updated software packages.
- Many improvements to the ZFS filesystem, including feature flags, TRIM support, enhanced drive removal notification, improved ARC memory reclamation and reliability improvements.
- Improved Plugin Jail subsystem which supports multiple jails and an enhanced UI including enhancements from PC-BSD Warden.
- Improved Volume manager including auto optimization of volumes for performance.
- Improvements to the encryption subsystem.
- Documentation enhancements.
As always, beta versions are for testing purposes only. Feel free to notify the FreeNAS team of any bugs you may find.