It saves its state like other multitask-optimized apps, so it'll present the last file you were browsing at launch. Also, you can open compatible documents for iBooks, Pages, GoodReader, or other apps that support the "Open in..." feature. The iPad's split screen display also makes it very easy to view documents while browsing folders.
If you have audio files stored on your iDisk, the app will let you listen to them in the background while you do other things.
MobileMe iDisk is free, but it requires a valid MobileMe account. The app was first released last year for the iPhone, and this is its first significant upgrade since last December.