The Galaxy Tab USB adaptor is small - about 25mm x 40mm - with a Galaxy Tab connector on one end and a USB A socket on the other. I've tried the adaptor with USB memory sticks, mice, and a keyboard - all worked except for the keyboard which the Galaxy Tab reports as a high power device.
Plugging a mouse in causes a mouse pointer to appear on the screen - and from that point on its just like using a mouse on a regular computer. You'll probably want your tablet in a stand for this - either the tablet is (nearly) flat on a table or your knees right in front of you for touch use, or it is on a stand so that there is space in front to use a mouse. I use a Griffin A-Frame
which is designed for the iPad but works just fine for the Galaxy Tab in Portrait or Landscape, even with the USB lead connected.
Plugging in a USB memory stick causes the stick to be mounted on the tablet's file system. The Galaxy Tab reports the connection and leaves a USB symbol in the lower-right corner of the screen. To unmount the memory stick you must touch this symbol and then click on the pop-up. Once the memory stick is unmounted you get a message telling you that it is safe to remove it. While mounted you can browse the contents with applications like AndExplorer or FileExport. If you put media on the memory stick then the media scanner will (eventually) pick it up and make it available to the appropriate media player applications.
Note that the USB socket is very stiff, so I recommend you remove the adaptor from the tablet before plugging in or removing anything from the socket.
The only disadvantage with this is that there is no way to supply power for high power USB devices, which in turn severely restricts what you can connect. It may be possible to plug in a powered USB hub and then plug high-power devices into that, but I've not tried this and of course you won't want to lug a powered USB hub around with you. A solution that allowed you to use the Galaxy Tab's own charger would have ben really neat - and for that reason I'm only giving four stars.