Siddhis are gained during the course of Meditation / Dhyana / Raja Yoga.
Its not easy to Gain siddhis. It takes years may be decades of Meditation to gain Siddhis. I am sure by that time you would have reflected enough to understand pros/cons of using Siddhis.
It is not good to use siddhis without understanding the consequences.
No action is good from every one. An action you take which may benifit one person may have a negetive effect on some one else. And especially with siddhis if you act irresponsibly you may do a lot of damage unknowingly.
Using Siddhis for wrong purpose will create bad Karma and it can have a negetive impact your spritual growth.
Siddhis Basi info :