They generally seem gimmicky, and are usually more about branding than anything else. Typically, you can play just as well with regular dice, though if used thoughtfully they can make certain results more obvious and save a bit of thinking. And sometimes they look kinda cool. So I'm not passionate about them, but I basically agree with MGibster that, all else being equal, they are a point against the game.
Mostly I just don't like having to keep track of the weird dice and remember which game they are for.