Brilliant Tutorials are one of the oldest in this. I had taken their IIT Elite tutorials and it really felt good. Their structure of the study guide is nice. Also the material is worth. However a person should have basics clear before approaching these because these are just additional material but not the only material that will be required for an exam. However one may feel difficulty in clearing the doubts as their response is bit slow.
Hence I would recommend Brilliant Tutorials as an additional material for preparing IIT but not as the only material. I would also advise to go through the whole material at least twice.