SuperPreview is disappointing, IMHO. It shows screenshots of the various browser's renderings, but they aren't interactive. Since a lot of cross-browser bugs are JS related that doesn't really help.
It lets you run all versions of IE, Opera, Firefox, and Safari without having to install them. They were also sharp and included the IE Developer Toolbar in the packages. Highly recommended.
I think that the browser sandbox and SuperPreview serve different purposes. The sandbox will allow you to do the js checking, but SuperPreview allows you to look more at design elements through overlay and rulers. This is the same feature provided in Adobe browserlab (mentioned in another comment), but browserlab is currently closed to new users.
Xenocode's browser sandbox on the other hand is great: http://www.xenocode.com/Browsers/
It lets you run all versions of IE, Opera, Firefox, and Safari without having to install them. They were also sharp and included the IE Developer Toolbar in the packages. Highly recommended.