Based on my personal experience, following are the pros and cons:
Pros:
Best, most dedicated faculties in the field.
Good coordination with schools. School portion was covered timely and properly.
Their claims about results are entirely honest (a rare thing in this field). Initially I was more impressed with the results of FIIT-JEE but a closer inspection proved otherwise. Vidyamandirs results are from Delhi alone whereas FIIT-JEE collects results from all over India. Vidyamandir results are purely from 2 year classroom coaching. No small print (4 day crash course, postal course, open test).
Overall they have very strong teaching ethics which I think is very important in this field.
They do not have a corporate culture, which is followed in some of the other coachings. The atmosphere is comfortable, homely and conducive for learning.
CONS:
They can be a little strict sometimes, especially if you are not up to date with your homework.
Join Vidyamandir only if you are serious about IIT JEE else you will find it difficult.