According to Seth Stevenson of Slate, the alien anti-pot ads are the best anti-drug ads ever.
Now, I've seen these ads, and I agree that they don't commit either of the cardinal sins of drug ads:
1) no melodramatic scare tactics (accidentally shooting your friend, running someone over, etc)
2) teenagers aren't really worried that sitting on a couch for a long time will ruin their lives
However, nor do I believe that they are effective. He suggests that the ads "pick at the insecurities" of pot smokers, that pot makes them less interesting, and less desirable to be around. I disagree. I watched the ad and thought "only someone who is currently high can understand this" or "I am not sure that I'd leave my boyfriend for an alien, even if he smoked pot all the time."
I don't know that many kids who would be disappointed in their friends for smoking pot. I also don't know that many who smoke around their significant others, if that person has expressed a preference for a pot-free environment. Granted, these are anecdotal points, but the ads just seem silly and bizarre.
None of the pot-smokers that I know have been in any way affected by these ads. Except to say "wow, the creators must have been on pot."