The iPad is good for consuming media (streaming or itunes), basic web browsing and email.
It's not good for "hardcore" things such as media authoring, most gaming, development or even advanced email/doc editing (no file manager, etc).
It can replace a computer for 90% of people out there in the world who only do the things its good at. It can also supplement a computer for everyone else - provided you find the right apps. But for a power user it will never replace any computer - including the Mac Mini.