Gah! Too many colons. And almost a week late now. Oh well.
Let's get this out of the way: the new Tron movie is not hard science fiction. If you have a background in computer science, you will smile at an open source software debate early on, and you might nod knowingly when you see that the Grid runs on some flavor of *nix. Beyond that, forget about TCP/IP, binary, or even the laws of physics. Tron: Legacy might look like science fiction, but in reality it's pure glow-in-the-dark fantasy.
The movie takes place well after the first Tron. Flynn, the star of the original, has been missing for twenty years. A mysterious page (it's like a primitive text message) draws his son, Sam, to his father's old arcade, where he gets sucked into the computer-generated world of the Grid where his father is trapped. Now Sam will have to rescue Flynn from his own out-of-control program.
Tron: Legacy is a gorgeous movie. The CGI isn't groundbreaking, but it's excellently done. Jeff Bridges is de-aged for a large part of the movie: his eyes aren't quite right, but for the most part you barely notice that it's a special effect. The action scenes are a treat to look at, as is most of the scenery; the costumes vary, but most of them look great.
The story is good. Not excellent, but good. There are plot holes in the movie that you could drive a lightcycle through: nothing that comes about through stupidity, really, but just things that never get explained that ought to be.
All of the characters are fun to watch - there's nobody in the movie you'd really want to punch in the face, except Castor, and he's supposed to be like that. Unfortunately there are two characters who represent horribly wasted opportunities in the film: without spoiling anything, one is played by Cillian Murphy (which means Disney is wasting a great actor as well), and the other is a program named Rinzler. I expect the movie ran afoul of time constraints, and I'm hoping an extended DVD release will do a better job of using these guys.
Overall I'd recommend Tron: Legacy as a good family-friendly popcorn flick. If you're a hardcore geek with standards, you'll probably want to pass it up. Otherwise, enjoy.