Fact or Myth - If you Sandbox Your Browser Your Completely Safe

(Background: I run WSA and the commercial edition of Sandboxie in addition to several virtual machines)
 
For all practical purposes no general use computer is ever safe. And for many users, sandboxing is not a plug and play, silver-bullet  technology - it requires you understand what they do and follow certain rules for them to be most effective. A sandbox is a tool not a solution.
 
WSA employs limited sandboxing against unknown executables in addition to journaling. In this regard it's above arguments about "antivirus is worthless because it only detects common things with signatures generated." However, it's not a hard sandbox that can be directly controlled or monitored. Other sandboxes out there will isolate a program from making any writes outside of its box which will protect the rest of the system.
 
The weakness in running most sandboxes is that they only block writes and also reads to certain sensitive areas - they usually don't block reads of your tax documents or files or programs. Just running a sandbox doesn't mean you're now free to let anything you find on the internet run forever inside that box.
 
If you're willing to split your life up into multiple sandboxes that you have customized to specially store data, limit reading, and delete the state after use, you can run a very tight ship when it comes to computer security that exceeds the protection of antivirus for almost anybody. However, in my opinion you still want the AV protecting Windows in case you screw up or something gets past/comes in a vector you didn't design for. Not everything can be sandboxed. Considering the light footprint and cost of a modern AV compared to the information you're trying to protect, there aren't very many great arguments for not running antivirus.
 
As with anything computer security related, this is an extremely complicated topic and this post glosses over many things. I'm just saying for the average user you still want the AV.
 
(Side note: If you're hardcore about sandboxing, check out http://qubes-os.org/trac which instead of running software sandboxes, runs entire operating systems)