Keep in mind that TV's don't make good monitors. Is your plasma 1080p?
A lot of people here use them as home theatre PCs/media centers. Using Frontrow or Plex, they offer a very cheap and advanced system for organising digital media.
In terms of use as a personal computer, the mini is ideal if you already have a monitor, mouse and keyboard. Otherwise, an iMac is probably a better option in terms of value.