To Sanctum's credit, the film features beautiful cinematography. The colors are really nice and the scenes all seem to be very well-lit. That's essential for 3D. If you have a good 3D theater near you (as some don't run their projectors at full power to save money), it might be worth going for the visuals alone.
However, the story seems to be filled with cliches. One of the characters actually says, "What could possibly go wrong diving in caves?" before jumping into the cave system. The survivors of the initial flooding seem to turn on each other almost immediately. Flaming torch fight? Eye rolls and sarcasm over a possible escape route? Tank-less long distance dives? What is this--an unofficial remake of The Poseidon Adventure?
What gives me some hope is James Cameron's stamp of approval. If nothing else, the man knows how important spectacle is to an epic film. Avatar is nothing without the gorgeous visuals that tell the story in spite of clunky dialog and too familiar story arcs. While Cameron did not make the film, he did step on board like other famous directors--Quentin Tarantino and Hero, Guillermo del Toro and District 9--to promote this film. If the visuals and action are strong enough to bring Cameron on board, Sanctum should deliver enough visual pop to create a memorable experience.
But, with a ticket price four dollars higher than a non-3D film, does a 3D action movie need to give you more of everything to justify the purchase? Sanctum looks like it's immersive (distance) 3D, rather than gimmicky pop at your eyes 3D. I just can't tell if there's enough story here to justify the extra ticket price.
Should you see Sanctum? If you value story and character higher than fancy visuals and loud booming soundtracks, probably not. If you can overlook the plot deficits, you'll probably have a great 3D experience. It just won't necessarily be because of a great movie.