So what's good? Lots. That first third of the film that's all Riddick is up there with those middle 20 minutes of Cast Away where Tom Hanks says nothing - it's a manly man survivalist adventure to make John Milius proud, and a testament to the fact that Diesel could have been a capable new Conan but for Arnold un-retiring and wanting to play him old. The main evil creatures are fun for a while - imagine a giant Alien chestburster on two legs with a scorpion's tail - though they get tedious en masse toward the end. And the sheer sleaziness of the whole thing is part of the point; I feel like I want to see more of this universe, but then I realize I did, in the overblown second installment. Snapshot POVs from the scum of the galaxy turn out to be more valuable.
Do I recommend this? I really don't know. As much as moments of the first act are considerably enhanced by the big screen, I feel like saying this is best engaged as a cheap rental. Should this universe get expanded on the creators' terms, I'm game for more - but would prefer it not be yet another variation on monsters in the dark with a token blonde.