*MINOR SPOILER AHEAD*
I saw this movie at a preview screening last week.
It was extremely disappointing.
The 3D - what there was of it - was stunning. Jeff Bridges very entertaining in both of his parts, and Olivia Wilde gave a good comic turn. And the action scenes were passable (although reliant on slow-motion at times.)
However, the script was a mess: badly structured (overlong lead up to the world); no connections between the real world and the virtual; it lifted its structure and sequences directly from Tron and other movies (Star Wars: A New Hope for one - Mos Eisley bar, Bridges is Obi Wan Kenobi, and there's a dogfight with spacecraft); the virtual world made no sense (how and why would it have wind and weather?); there were plenty of overlong lectures and monologues; there were basic flaws in logic (for a start how does someone bleed in a virtual world?) and continuity (at one point a character has their data disc stolen. A few minutes later it's clearly seen on their back, when they're discussing how to recover it); and characters behave in contradictory ways.
The other major flaw was lead actor Garrett Hedlund. He just does not hold up as a lead in this film. Especially not when he's stood next to Bridges and Wilde.
The biggest flaw was the lack of Tron. The lead character from the last film is now a (literally) faceless, generic goon. When Bridges character recognises him as Tron, it's hard to see how - Tron looks identical to every other goon in the film.
Watch the original. Tron Legacy is entirely skippable.
EDIT: @tronprogam2 has pointed out that the disc I mentioned earlier belongs to another character. So that continuity flaw isn't really a flaw. But I found it hard to pay attention as I was very ungripped by the film. And that particular disc comes to a grim end at the end of the film, which is another part to characters that doesn't make any sense.
Also, there's a blatant "Reason for the inevitable sequel" moment near the end.