I am holding off on the ZFS. I Should have held out on the jails as well. The "upgrade" button next to the jails were just begging me to push them. I am a fat guy so you know I don't have any self control. But the jails are not a deal breaker so, whatever. So far so good.
Couple things. I was told the upgrade to 9.3 works better from 9.2.1.9 . I see you are currently running 9.2.1.9 now. I was not so I did a double upgrade. Second off, make sure you download your configuration file and your volume keys to your desktop (windows or whatever) before the upgrade. But you may be able to skip this step if you are not stepping up to a different thumb drive and it is just a simple upgrade using the same drive. Pretty much a piece of cake.
I did have to boot back to 9.2.1.9 a few times to get my volume key to my jail volume that I don't ever remember making encrypted. FreeNAS reported it as a locked volume. But it taught me a bit about how keys work and kept my google skills honed. So I know reverting back is as simple as a thumb drive swap.
Another issue I ran into. FreeNAS was flashing yellow about the drivers for my M1015. As far as I know, there is no fix for the problem. I just unchecked the box so I can bury my head in the sand.
FreeNAS also flashed yellow wanting me to upgrade the ZFS pools. But you we have commitment issues about that. Unchecked those boxes too.
The Wizard sets up basic stuff and helps you set up pools. If you are starting a fresh machine it seems to be useful. For upgrades, it doesn't do much of anything so I just closed it. If you ever want it back, there is a button on the top bar of the GUI. That is cool so it is not gone forever.
I even got ambitious and installed Windows 7 in a virtual box jail. I have no idea what I am doing. Standby for a "I broke my server and hosed all my data" thread.
Go for it.