Whilst this is meant to isolate your browser from the rest of the system, most people then want to be able to allow it to download/save files/bookmarks, etc. to certain areas, this in effect removes some of that isolation and could well be opening the system up to potential attack.
For many running a browser in a sandbox is adding another level of complexity and uses more system resources as you have to be running another application to handle it.
Personally I have never felt that it is necessary, I have got by without it without any virus issues until now so I don't see it as a panacea to resolve all your problems. I thought about using something like sandboxie for a long time and that is as far as it ever got, I downloaded it and never bothered trying it.
I prefer opting for pro-active measures rather than introduce a sandbox, I always run all internet facing applications, browser, email clients, etc. using DropMyRights this restricts the browsers right to that of a limited user (only works up to XP). This limits the potential for damage should you ever did get hit.
I ensure my browser is fully up to date, I run firefox with some security add-ons, NoScript and ReguestPolicy, these block all scripts and cross site scripting unless permission is given.
Then I obviously have avast's Web/Network Shields and the File System Shield.
Then as the final fall back - I have a robust back-up and recovery strategy (hard disk imaging software), should I ever experience a system problem, no matter what the reason. So I feel relatively secure without introducing sandboxing.