Yeah mate, usb2.0 will not be as fast as an internal, but as kosmiq said they are very handy, i use a USB2 maxtor barracuda 200gb, ideal for transferring data between pcs without wasting discs, also VERY handy when reinstalling your operating system!! Just drop everything you want to keep onto the HD then wipe your PC, when you have finished just recconect and drag it all back to it's original location!!
Thats the main use of mine bud oh and my ipod music collection, couldn't face having to redo that.
Definately would not be without, think i paid £80 inc p&p for mine from ebay!