Fair enough. It does not seem to have undergone the level of scrutiny that one would hope for. But for me it seems to make sense to trust one app rather than having to individually trust the dozens of random apps i encounter day-to-day use. I am pasting an example i gave in a reply to another comment :

"So for example the other day i wanted to download a 60fps video from youtube, and the only practical option was to use and adware ridden java downloader.. So i just downloaded it, created a new sandbox (based on the template sandbox) for the app, installed it (i recall i had some minor kinks in the install process but managed to get it to work in the end); after i was done with it , i deleted the sandbox, and i was off on my way without having to worry if the application had left some unsavoury bits on my system.."

edit: It seems Sandboxie was acquired by Sophos, so now you have to decide whether you trust Sophos or not.