I was in Resonance Kolkata in VIJAY batch of 2015-16. I had dropped a year off to prepare for IIT and other entrance exams. Resonance is quite a big name now but all that name and fame is mostly associated with the main Kota branch.
The good thing is that you will get the same study material as everyone in India. The material is quite good if you have the time to go through all of the DPP and the theory part.
The teachers are not very good at explaining the concepts. During my time, they didnt even have a chemistry teacher for a long time. Whoever came, used to leave in a week or two. The final one we got was very inexperienced and not very knowledgeable. The only decent teacher was the Physics sir who constantly motivated us. They do clear our doubts but like I had said, they are not very knowledgeable.
The teachers speak very poor English and the administration is not upto the mark. They dont listen to the complaints of the students and take long time to get better replacements of the teachers.
Joining the institute is not bad but its you who has to do the hard work to get to the big colleges.
Cheers!