A cult is no more or less valid then any other religion in terms of doctrine or right to truth. In essence a cult is simply a religion with fewer numbers and therefore less social exposure than a mainstream religion.
This means it's members will have a greater sense of social isolation, since they're a member of a smaller alienated group. Since this would be a powerful demotivator (stigma from this is going to cause family members and loved ones to persuade them to leave the cult), the cult will use extremely 'persuasive methods' to keep members in the flock, such beratement during public speaking (in front of a group), as well as other methods to crush people's self-confidence and ease the difficulties in the indoctrination process. Often things such as laughter (during a weekend retreat for example), are only permitted at the end of a retreat (to symbolise a form of 'newfound happiness'
Cults can also gain some form of legitimacy, unfortunately the I.R.S couldn't crush Scientology (notorious for it's exploitation of rank and file members) back in the 80's. So now it's in the process of gaining legitimacy, through exposure and growing numbers. Whether a cult is harmless (Hari-Krishna's seem odd but harmless) or malevolent (Branch Davidians was it?) outside of what it does to your social life, is judged on a case by case basis.