Touch is handier (geddit?) but much less precise that the mouse.

• It depends a lot on what you are doing. For browsing it's convenient, but you can't do any detailed work with touch screens
• It depends on your posture. Touch isn't very useful when your computer is on a desk, but great when you are lying on a couch. If you use the same device in both positions it's a good addition.
• It depends on what you are used to. I have a touch screen mostly for casual use, but find myself occasionally touching my non-touchscreen since I started using the touch screen.