The story of Sam Fisher turned out to be wildly popular, moreso than I think Ubisoft anticipated. Enough for a limited collector's edition to be released (only on the XBox 360) with a nifty sleeve cover and a variety of info features.
The limited edition has a documentary on covert ops, another documentary on the making of the game, multiplayer tips, mini films, trailers, concept art and an interview with the voice actor for Sam Fisher, Michael Ironside.
Is the extra money worth it? That point is moot actually, because the regular and limited edition were both the same price, so we bought the one with extra features. I think we've watched them once or twice. Unfortunately the game just isn't as good as expected.
There are some major benefits - you can save anywhere, for example, and the graphics are better - but the storyline isn't, and there have been glitches in the game that required patches on XBox's side to fix. For me, this game is an example of one of the 360's many problems.
The price has come down significantly now though, so I wouldn't feel as bad about paying $20 or $30 for this game.