Someone on JBL suggested cutting/sanding down the front piece on the crotch (
); I did that to my set, and it improved their mobility immensely (you have to take off a pretty good chunk of plastic, but it's not noticable once the figure is back together). Out of the box they're basically unusable.
Their heads are also problematic; there's a tab in the back of the torso, right in the collar, that kills any hope of posing. Once again, you can sand or cut the tab off; the head doesn't end up too loose, but if you get it to the point where it's fully posable, it's definitely not as tight as a new figure should be.
They really screwed the pooch on the Viper Pit figures, but with a little work they can be decent.