So, I saw it last night and had a somewhat more positive reaction, though I was hardly blown away.
Now, for fairness sake, we did see it in the Sydney IMAX theater which is the largest IMAX screen in the world, and that contributed quite a lot to the experience. My overall impression was: cool visual spectacle and entertaining, but braindead, action flick. And, to quote my girlfriend: at least it was better than Avatar (admittedly, that's setting the bar pretty low).
The thing is, the original TRON, which I saw recently, was also pretty badly plotted and had paper-thin characters. What made TRON special was not the storyline, but the effects which were amazing for early 80s tech. TRON: legacy has all the same issues, but the effects, while cool, in no way exceed our expectations.
(Though, there are cringe-worthy moments in this one, esp. the most predictable betrayal ever, plus the most obvious fate for the traitor ever, that are actually terrible plotting, not just two-dimensional plotting. But luckily they are over with relatively quickly).
My advice is the opposite of Bdoomed - see this in a big screen theater with 3D, so you can enjoy the visuals. Forget about renting it, as on a small screen it will lose most of its entertainment value.