You get a huge thumbnail of the destination window; it's far larger than the one that comes with the "stock" Alt-Tab. Another neat feature lets you Alt-Tab between the windows of just one application. It's kind of like pressing Win+[number] in Windows 7, but you don't have to look at the taskbar and count the icons in order to see what the number is for the application that you want to switch to. That's done using Alt+` (backtick) by default, but it's configurable.
Read more: DownloadSquad
Read more: VistaSwitcher