The Do Not Call Registry works perfectly well at what it was intended to do, stopping calls from legitimate telemarketers. It doesn't stop scammers and other crooks any more than laws against breaking and entry stop burglars.
Putting your number into the Do Not Call Registry is always a good idea. Even though it doesn't stop crooks from calling it does insure that no one other than crooks will make sales calls. In other words, you never have to wonder if a telemarketing call is legit – if you're on the DNC Registry you *know* its a scam call (however much the creeps on the other end of the line try to deny it).