As others have said, its definitely sinister, and dangerous.
1. It has money - lots of money, and is an extremely litigious organisation
2. The core membership, seaorg, is run on an almost military basis.
3 Members of Scientology are discouraged from mixing with non- believers, and there have been several documented accounts of members ostracizing their family, causing pain all around
4. They have some strange beliefs about pyschiatry, counselling, and mental illness.
5. They can be very damaging to your health. Members are pressured into parting with lots of money to advance through the different stages of development - thousands of dollars.
6. They use a patented device called an e-meter to scan various supplicants within the religion. Despite losing a court case, so now they have a legal obligation to state that the object has a purely religious value, their literature and processes still encourage the idea that such meters have a medical or scientific value in treating or diagnosing illnesses.
7. Infiltration of government and other institutions in the 1970s. Scientology infiltrated hundreds of government agencies in an effort to trace and steal unfavourable documentation about L Ron Hubbard and Scientology itself, including phone tapping. 11 highly placed Scientology executives, including the then wife of L Ron Hubbard, were found guilty of obstructing justice, theft of government documents,and burglary of govt. offices.
There has been anecdotal evidence of physical beatings of members wavering in their conviction to the Church, of mental torture and solitary confinement, harassment of ex- or apostate members, and more.
Despite all of this background, they are still treated by the US Govt as a genuine religion, and as a charity with all the tax benefits that brings.