Some of the shows listed in the TV directory (Comedy Central shows, for example) load it in a browser, and you have to manually maneuver a cursor to the fullscreen button. One of the Apps on the box is a simple web browser, that would allow you to navigate anywhere.
That said, Boxee has small handful of shows/networks that automatically switch to fullscreen for you – but those seem few and far between.
I used to run boxee from a macbookpro onto my main television in the living room. This worked surprisingly well alongside either the Apple remote or Boxee Controller App.
Moving and re-hooking up my main computer to watch media was obviously tedious, so I ordered a Boxee Box at launch.
The Boxee Box simplified the hell out of my media experience, which is definitely worth the $200 price tag. A laptop would probably work just fine, but the Box has the new 1.0 software, and the remote included is pretty great.