Just as with the website and its browser toolbar, the app works quite well and quickly to display photos, pages and videos on your device whenever you touch the "stumble" button. Some of the icons aren't obvious as to what they do, such as the one identifying items in your StumbleUpon incoming links box. Other than that, though, it's a well-polished interface.
Something the app does not do, though, is allow one to submit new links. For that you will need to use a bookmarklet with Safari or use your desktop browser. The app is available now, for free, in the app store.