Whereas the Preview Pane is awkward to reach (having no hotkey) and awkward to use, the ShellToys Preview tool opens in a separate window and can be assigned a hotkey for quick access.
Otherwise, Preview uses the same previewing technology as Vista: if a file type can be previewed in Vista's Preview Pane, it can automatically be previewed in the ShellToys Preview window.
If no preview handler has been registered for the chosen file, Preview will offer you the option of previewing it as text, or using any other installed handler.
ShellToys also maintains a list of custom preview handlers you've defined. For example, you can specify that CSS/INF/REG files should be previewed using the plain text handler, and ShellToys will automatically use that handler in future for previewing files of those types.