Iam learning from byjus app and iam going to write jee in 2017. their explanation and visualisation is good but you cannot just depend on only those vedios for cracking jee. for chemistry it is good,but for maths and physics, it is useful to make your basics stronger. but you need to practice a lot of problems for maths and physics.They provide you mock tests and chapters tests too. finally I would recommend u to take byjus app if your not good at your basics and it's good to buy its application if you want to start it from the scratch,other wise no need ! All the best !